At the time, I was young developer being asked to interview a somebody. If I'm honest, I was a little prideful that I was interviewing him, and not one of the other, more mature developers, so I took him through the process.

I think, knowing what I know now, and feeling confident in who I am as a developer, I'd probably at least ask about it in the interview. I'd say "Why did you choose to wear that shirt today?" Serious question. I'd be interested. Maybe the answer is "Because I don't really want this job" and then it's clear. Maybe it's "Because I don't want to work at a place that gets offended easily" or maybe it's "Because I didn't have any other clean laundry." I cannot think of a valid reason to wear that shirt to an interview, but I'd at least be interested in his reasoning (though he's still not getting the job).