Is it really that black-and-white? It certainly has at least some characteristics of charity, such as being non-compulsory, and often done out of a sense of goodwill.
The busker is there doing his thing, he's not asking for money to play. He's working though, and payment acknowledges the value of his work.
Charity, on the other hand, is supposed to expect no work in return. You help people whether they did some work or not.
The fact that the OSS dev didn't charge for the software in the first place doesn't mean paying him for that software is charity. It's acknowledgement, and a possible investment if you depend on the software he provides.