Money
Laundering,
rings a bell ?
They are popping up because it is good, mostly passive income if you are in the right area.
(Edit: maybe "sure sign" is a little bit hasty, but I think the other possible reasons for a "credit card only" sign are actually worse morally than money laundering)
Maybe some tax avoidance scheme, not money laundering.
Unless you advertise as credit card to outsiders but take cash as part of some scheme