Have you played it? That combination makes Bannerfall way better. I've seen people that always or mostly play on premade teams say Bannerfall is a really good map and its easily my least favorite trials map so I never understood how they could think that. But the combination of the zone and freelance definitely makes it 10x better. Not sure it would be quite as good without the zone and its probably helped by it more than any other map I've played. Or it could just be me that's helped by it but that map for me has a problem with certain things feeling oppressive that may not really be problematic otherwise. Like snipers, there's no problem with snipers but when I play on that stage normally I get the "I just don't feel like doing this" feeling because people will just be standing there and making it so you can't go this way or can't go that way. So then you're stuck switching to one and trying to do sniper ok corral which I'm not that big of a fan of either. Then while you're doing that there's fusions and titans ballistic slamming... Long story short I really think the zone helps.

Why not just kill the enemy & not have to worry about zone?

Probably because they might not be Available Next Season/Exspansion. Doing 1 Last "Survey" at the same time. They might do 2 Different Labs next Season.