If you were asking me, it is 1) because the airlines are regulated and there are some in the FAA/TSA that would make life very difficult if they pushed back, and 2) because it really does lift potential liability. With respect to point 2), to get that relief from liability they have to give something up. Add the fact that they don't want to be publically portrayed as "lax on security", and you get a "no pushback" environment.