The fewer signs there are the more plausible deniability we have. There is a general push to avoid places that try to prohibit firearms, but sometimes that isn't so easy for everyone. In those instances, it's nice to still be able to protect yourself. Not every employee at every location pays attention or even knows the rules, so it's still possible to carry. Hence the difference of my experience.
Also, for non-PFZ's there is still the option of concealing.