Strangely they seemed to operate within the law
Yep - in London they're 'near beer' clip joints, serving beer under 2% ABV or weak fizzy wine, for which they don't need a drinks license. It adds insult to injury for the poor suckers that get scammed that they're paying £50 a pint for 2% ...but the tariff is on display (in small letters, in a dimly lit corner) so it's legal. As the girls don't actually promise anything they don't deliver, it's not deception, either.
It plays on male vanity and that blokes seem to lose all common sense at a sniff of skirt. Common in the 'party' destinations - Prague, Budapest - I guess it was only a matter of time before it reached Krakow. You can be fairly sure that others will follow.
While I empathise with the guys who get scammed, clipping has its funny side. I was a member of a drinking club in Soho - the Colony Room - with the entrance at the top of a dingy flight of stairs from the street, not much different from the entrances to flats used by 'working girls'. Clippers would take money from punters on the street and tell them to "Go straight up, dearie, the girl's waiting for you". As the poor punter opened the door and walked into a private bar full of artists and intellectuals and their sad little faces changed from lust to shock to realisation and to shame we couldn't feel anything but sympathy for them. The humour came from the very few who refused to believe they'd been scammed and kept asking 'where's the girl?'