As Vecomti said, people who have had experience on other Jailbreak servers would feel alienated by being teambanned as soon as they join for the very first time, and might get the false impression they messed up and go hunting for an admin. There would be way too many complaints from newbies about being teambanned.
- There might be occasions (as uncommon as it may be) where the overwhelming majority of the server are newbies and thus severely limiting the people who can go on BLU.
- As for newbies who intend to join a server just to mass freekill, I'm pretty sure an "introductory" teamban won't stop them, once the teamban expires they'll probably get right to mass freekilling as if the teamban never happened.
Simply put, most newbies who join BLU right away will probably break a few rules, yes, but once they are reminded they typically remember them and try to abide by them. Contrary to what so many people believe, I've found it's uncommon -- or at least, not rampant -- for new BLUs to completely ignore any and all reminders they're given if they break rules, and if they do so, there's a high chance they might be trolling. People seem to like to paint all newbies to BLU as oblivious to the rules and completely ignorant of their surroundings when I've found that's usually not the case.