In the United States, non-US citizens entering the country for work purposes are generally required to get a visa. There are a few (and very few at that) business exceptions to this process, as listed here: http://travel.state.gov/visa/temp/without/without_1990.html
He stated that he was: Trying to work several unpaid shows, he would receive meals and tips as compensation at one or more shows, and then he was going to write articles for Noisey (http://noisey.vice.com/) about it.
Bringing musical instruments with you (and admitting you are a musician - even if just as a 'hobby') on a supposed tourism visit and going to one or more cities where you did not have relatives or explicit non-business reasons to be there probably immediately triggered an investigation as to why he did not have a visa / whether he required a visa for his visit.
Yes - our border control system is horrible. I totally grant that. But this person was also explicitly not playing by the work visa rules either.