First of all, it's FRED. Expect the unexpected.
Secondly, I think there's actually an entry in mantis on this. The reasoning for it is, you shouldn't need hi-poly ships for designing the mission, a more responsive FRED is more important. There was talk about fixering up FRED2 so you could select, or it automatically switch, LODs, but generally getting FRED2 working is much higher up on the list.
If it bothers you so much, you can always make yourself a set of FRED2 models in a VP, and switch that in when you want to use FRED (or put it in a directory and use a special -mod command with FRED, and the multi-mod feature with fs2_open

)