Sitting here waiting for my flight, it occurred to me that although marijuana is legal in my state, and is also legal in my destination state, it is still illegal to take some along for the trip. Does the law change with altitude, or is it because I'll be passing over Utah?
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The blue meanies pop out of the over head compartments the minute you cross over into Utah and arrest you.

Likely because you are crossing state lines. All things in America seem to get serious where state lines are concerned.
 
......Or so I'm told. 

Um, there are plenty of states where it's illegal at any altitude if it's consistency you're looking for.

IMHO and I don't partake though it's legal here,  making the evil weed a schedule 1 drug is like calling spam ham, no?  Think of all the unnecessary suffering this has caused!  I'm beside myself!

My guess is (as mentioned) the crossing of state lines.  ANY state lines.  As soon as that happens it becomes under Federal laws which, still, have not legalized the wacky weed.
 
On a related note, my ex-roommate was applying for a job where his "work area" (as a service technician) consisted of parts of Oregon and Washington (basically the so-called Portland Metro Area).  He applied, was interviewed twice and they wanted him to work for them, but he cancelled the process when he found out they do random drug testing which STILL includes marijuana as an illegal drug because the company's headquarters were in some mid-west state that uses the Federal guideline.
 
I've never smoked anything, but even I think this is just plain stupid. 

Actually, it is because you are traveling on an aircraft subject to federal security/screening which puts you at risk for arrest under the Federal law that already makes it illegal to possess cannabis in our state, and specific federal law against transporting cannabis by air. Cannabis is a schedule 1 drug under federal law, and there is nothing preventing the DEA or FBI from breaking down your door in Washington and hauling you off to federal prison except for a sort of policy of forbearance that they will not make that a priority. That arrangement is totally dependent on who is elected to head the executive of the federal government, and to a lesser extent to the federal prosecutor in your district, who retains the power to prosecute regardless. The policy, by the way, does not apply to primary federal jurisdictions within the state, so National Parks, military bases etc. are not safe even within the state. 
https://www.justice.gov/iso/opa/resources/3052013829132756857467.pdf
 
 
A federal law does block the DOJ from using funding to interfere with States legalizing medical marijuana, but prosecutions have nonetheless occurred and an appellate decision has not been rendered. DOJ interprets that wording to mean only that it cannot sue or attempt to enjoin the states themselves from legalizing medical marijuana, but does not affect criminal prosecutions of individuals in affected states. In any event no such wording applies to recreational use. 

craigb
On a related note, my ex-roommate was applying for a job where his "work area" (as a service technician) consisted of parts of Oregon and Washington (basically the so-called Portland Metro Area).  He applied, was interviewed twice and they wanted him to work for them, but he cancelled the process when he found out they do random drug testing which STILL includes marijuana as an illegal drug because the company's headquarters were in some mid-west state that uses the Federal guideline.

 
Actually, most businesses that test employees for drugs do not do so to be in compliance with some government guideline, but on their own initiative. They believe that employees that use drugs are of less value than those who do not, even if they are sober at the workplace. Casual drug use can be taken as a surrogate marker for irresponsibility, unreliability or poor productivity, and they do not care to examine, either the scientific basis of this prejudice or the specific behavior of a particular person when they are designing an employment policy. This is the same kind of thinking that leads them to do criminal background checks, credit reports (in states where this is legal) and require letters of recommendation. All things being equal they believe that an employee who does not use drugs or alcohol, pays all his bills on time, has a respected place in the community, and a stable family is less likely to experience disruption in his private life that may affect his work. They often justify this intrusion on the private lives of their employees as a matter of safety, and in this they are supported by their insurance companies, which sometimes either refuse to insure them or charge a higher premium to insure a workforce that is not drug tested. Of course the policy is often applied to all employees, not just those whose jobs actually involve a risk. The problem with cannabis testing, is that the positive test is poorly correlated with the level of capacity of the individual at the time of the test, since the drugs remains detectable for weeks after use, when the user is no longer under the influence.
 
Most states have no law to protect users from being fired for a positive test with the exception of medical marijuana use in a few jurisdictions. As a practical matter it is far easier to fire an employee because of a positive test violating employment policy, than it is to prove he is dangerous, incompetent or not productive. 
 

Good points!
 
I find it ironic that I've personally known or met so few of the so-called useless pot-heads you see on TV (typical media hype).  In fact, many of the brightest guys I know partake (and they drink less alcohol as well).