Above was written:
If he's not getting paid, which I assume is the case, shouldn't it be classed as a "personal interest" or "hobby" visit, and not need a work visa (let's ignore a few CD sales along the way, because they are probably for cash)?

Absolutely correct. If an artist is coming to the US only to do a benefit concert *for which s/he is not getting paid*, then there is no need for a work visa. The artist can come in on a tourist visa. There *may* however be questions asked at customs re the purpose of a guitar (or whatever musical instrument), so one needs to be ready to say "for personal use only" and also to declare the value of the instrument going in and out. Bringing CDs in for a "vacation" will also send up a flag; in the case of a benefit concert, these should be shipped ahead to the venue and held there for the artist's arrival.