More on the question of why the "hanging" Louis Riel t-shirts are racist:
I don't think you can say an image of a single person is explicitly racist. You can always come up with an alternate explanation--e.g., "I'm criticizing (joking about) this individual, not his entire race."
But I say such images are implicitly racist. Why? Because we don't see huge galleries of sociopolitical images with everyone's neck in a noose. No "hanging" t-shirts featuring George W. Bush, Hillary Clinton, Pope Benedict, Nancy Pelosi, Dick Cheney, Sarah Palin, Michael Moore, Rush Limbaugh, Al Gore, Paris Hilton, or whoever.
In short, these images target people of a particular race. Usually blacks, occasionally Indians. Almost never whites.
Show me a gallery of images where everyone's neck is in a noose--NooseShirts.com?--and I'll say they're stupid but not racist. Until then, let's get real. Nooses are a well-known way of sending a hate message to minorities.
A comparable case
Consider a comparable case: a cross-burning. You could say the perpetrators didn't burn a cross in someone's yard because he's black. They did so because he didn't mow his lawn, his dog barks too loud, etc. Even claiming he was "uppity" and didn't know his place could be construed as a nonracial message.
But if you look at the incident in context, you learn people don't address every social faux pas, black or white, with a burning cross. Historically, a burning cross sends a particular message about blacks not being welcome. About blacks being so unwelcome that the perpetrators can violate the law with impunity.
Because each new cross fits the pattern, it's presumed to be racist unless proven otherwise. Same with noose images such as the one on the Riel shirts. They're racist because they're part of a long series of racist messages involving nooses.
For more on the subject, see Highlights of the US Report to the UN on Racism.
Below: Just a harmless choice of clothing not directed at any particular race?