There's no good reason for not allowing conversions. They can have shorter cranks to compensate for lower BB (and a track bike can have a longer cranks, thus eliminating the advantage of a tall BB). A wheel falling out of a dropout is unlikely and even so, no less dangerous than the weel that wedges against the seat tube. I have a conversion that has a steeper geometry and tighter wheelbase than a lot of the so-called track bikes out there.

I think it's telling that UCI has no regulation about rear fork ends for track racing.