Remember I come from america too. That t-shirt doesn't say that to me, and it wouldn't have looooooong before I came to europe. Oh its rude and crude (derogatory), but it doesn't say anything about crimes against anyone. Its humor. Not everything has to be taken with a political perspective, and certainly not everyone in america does.
Kind of anal to think so too. Reminds me of a black guy I got stuck working with a while back in Harlem. Every other word out of his mouth was, "well the white man says, or the black man says". He had a reputation for that, and after about two weeks of working with him I had to tell him to stfu because, I never had no slaves, my daddy didn't have any slaves, and my grandfather never had a slave, and I think if you go back far enough we were probably slaves at one point.
It was so bad his racism that it was a long running joke at our job. If the boss asked us to paint a register white, or even mention the color black or white, we would ask, "whys it gotta be white?" "why cants it be black?" Both white and black people participated in this long running joke. I think it's been over twenty years now, and it still goes on.