I believe that its working as designed.
Since it has been like this since release.
Not 100% sure ofcourse, a dev would have to say.
I suspect they kept it that way because ost infantry can't fire out of the 251 HT whereas sov infantry can fire out of m3 and m5.
Ding ding ding! We have a winner here folks!
The key difference is that it was easier for Relic to make it so that Soviet MG's and Snipers couldn't enter their halftracks at all rather than coding that they can't fire out of them, I guess.
In any case the German halftrack, German snipers and MG's can't shoot out of them, they make for a great transport.
Fun fact, German MG and Snipers can't enter the open top 250 Halftrack that is doctrinal.