For the clubs, it's a licensing issue. When the club got its licenses, there will have been a definite 'no prostitution' clause, and possibly even a 'no touching' one. Some places turn a blinder eye to it happening than others...
For escorts, sex for cash is legal and there are no laws against it to 'get around', so - unless you're telling people about this on the street or other such public place - you don't need to pretend it's sex for companionship.