Ok I can understand your confusion and yes you are right in that tempo does need to change. Example: If you do a single bar of 2/4 time at 60 BPM then you will need two seconds to complete the bar. Now you can divide that 2/4 time up into two lots of 3 eighth notes.
In order to keep the timing per bar the same one has to now go up to 90 BPM of 6/8 time sig and you will still fit 6 eigth notes in and they will take the same time. EXCEPT you are feeling it differently now. You are feeling 3 groups of two eigtth notes but what you really want is to feel two groups of 3 eigth notes.
The way I would do it would be to select 2/4 time and set the grid up so it is showing eigth note triplets per quarter note. Hence you can insert notes etc and you will get the TRUE feeling for 6/8 time. 6/8 time is meant to be a faster 2/4 thing where you get the feeling of a fast 3/4 feel per beat.
(that is how my wife explained it to me and she is a trained classical musician) So yes DAW's get confused about how they are clicking off 6/8 time and I think it is good that this has been brought up. The problem is how the 6 eight notes are interpreted within a 6/8 bar.
That little video that was mentioned earlier is actually how 6/8 time is felt. The way to work out the tempo that is required is to feel the 6/8 as two beats of 3/8 and figure out what the 2/4 tempo has to be in order to make it happen.
To answer
Sharke's question below. A beat is always a quarter note and can be 2 eigth notes or 3eights notes as in the case of 6/8 time. (6/8= 2 beats of 3 eigth notes) You would need some tricky metronome options as mentioned below (now in PT) in order to be able to click this correctly.