Of course, users have the ability to edit their own thread titles so if someone thinks a problem has not in fact been solved, they can change it back (as you did).
But the problem is that a lot of users don't modify the thread titles. I read every thread that has a "solved" or "resolved" simply so I can be aware of problems people encounter, as well as the solutions. That has helped in terms of helping other users, but also, in terms of solving problems I encounter in my own system. Also, having "solved" makes it easier to do searches.
I marked your thread as "solved" because I duplicated the conditions you specified, was able to solve the problem, and gave the steps I took to solve the problem. So, it sure seemed solved as I was able to duplicate the problem and come up with a solution. However, I think that's the only instance where I've marked something in opposition to the sense of the poster. The forum is moving so fast these days that if thread titles aren't modified within hours, the thread will often slip off the page and no one will learn about a solution to a problem that might affect them.
The ability to scan which threads have solutions to problems is very important. If all users would do this, it would be very helpful but they don't. So, we have four choices:
1. Force people to edit their threads as appropriate (ain't gonna happen).
2. Let threads that solve problems and don't have thread titles edited exist in an unmodified state (diminishes the value of the forum).
3. Edit thread titles as appropriate (possibility for inaccurate assessment)
4. Ask for permission before modifying the title (increased workload, and I assume most people would say "ok" anyway)
All have pros and cons but given that people can edit their own thread titles if they feel there has been a problem, then (3) seems like the best option.