From my own personal experience, walk-ups charge extra for kissing if you are a new customer to them since they don't know you that well(they even have a price for it on their walls). Some will still charge for DFK if you are a
regular.
From what I can tell, its because they feel its more for love than PS/job so they try not to do it, naturally if theres a connection between the two of you, then its on!