Can we talk about comedy for a second? the top part of the image is fairly well known (I'd seen it before at least). It's supposed to be an inspirational message talking about the wonders of multiculturalism. It's perhaps a bit dodgy towards Mexicans.
The added, actually racist bit at the bottom is the punchline specifically because it is a bit racist. That why you laugh when you see it, your expectations were subverted, you were shocked just a tiny bit and that makes you laugh. A comedian, I can't remember who now because it was on a podcast about a year ago (Marc Maron maybe?), talked about comedy being a consensual violation and I think that's appropriate to this image. It's funny because it steps over that line of propriety that we set as a society. If you laugh, it doesn't mean you agree with the racism at the bottom, most likely the opposite in fact.
I think the worst thing about these stories is that they bring out the actual racists in frothy mouthed defense. Sad.