It is the maps...only Omaha allows you to continually respawn until time runs out or the objective is met. The others won't let you respawn until the end of the match.
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~Pvt MaNgE~
141st Infantry
Official BTFH
I believe there is a way by which the server can force respawns, but as far as preventing them, I'm not sure as of right now. I believe the intent is to simulate the conditions of the battle being reenacted. In the Omaha map for instance, I would think they idea is to simulate large numbers of infantry on both sides; of course with less than 30 players total this is all but impossible unless respawning is alowed. As far the other mp maps without respawning, I'm not sure if my reasoning stands up, but it is a logical guess.