I'm presuming those who say they allow 30 days mainly do bigger jobs , installs, bathrooms. It seems more reasonable to wait a while for a big payment for a big job (even though it's no different in my opinion.)
My problem comes when people ask me "How do you want paying?" And I mention I'm accept cash, cheque or online transfer. I don't ever remember saying I accept cash, cheque or pay in a few weeks if that's easier for you or you are prone to forgetting or can't really be bothered yet or would rather see how this weekend goes because the girls are over and someone mentioned a big one.