I've done busking in Europe and I was doing it for money for a few beers and to help out with some money and food for my flat mates - no more no less.
Put on a performance / sing the right songs the money will flood in .
After all , I wasn't mugging anyone, selling drugs or anything nasty , I already had this easy way of making a few Quid (Bucks) relatively quickly by making people feel good albeit for a short while - and I always tried to say (or show/indicate if in the middle of a song ) a thankyou for all contributions.
It's all a matter of fair exchange - nowadays, I always give to a decent busker or to the busker who looks as if he's /she's really trying to give the bypasser or listener some value or pleasure.
To the introverts and mumblers - I give nothing - but, I believe that trend existed when I was doing my bit on the pavements and sidewalks.