I think it is because the strikes take place on the patrol maps. There are certain areas, especially at the beginning where you just don't need to kill the enemies, there is no point to do it. I've always looked at that first group, before you get to the "no respawn" area, as being part of the patrol map you are on and not really part of the strike. It makes sense from an ammo conservation at that point as well. If you've run a strike more than once (and who hasn't), all you really want to do is get through it, collect any rewards, then get back in for the next one. At this point you aren't going to see anything new or exciting, it's all about efficiency. That's one of the problems with having such limited content.