My understnading is that part of this comes from the fact that many government installations have had barriers put in which MAKES EVERYONE ELSE AN EASIER TARGET (this is the supposition, I'm not sure I agree). There was quite a bit of liability protection put in place after 9/11, but it's widely believed to be a one-time deal. So building owners are worried that, if the govt. building down the block sets up a barrier and they don't, are they being negligent? Will they be taken to the cleaners later for not "getting the hint" ...?
So it's sort of a "we gotta do it because everyone else is doing it" scenario.
/jordan