This was on the TITP thread not too long ago. According to these idiots, "healthism" is a form of oppression. Depending on who you ask it could be because we are discriminating against those not in peak health, or that we're using archaic standards of health against people (which are apparently outdated and were found through sexist means or something)
Basically it's their way of ignoring anyone saying that being fat, especially being obese, is unhealthy and that that's a good reason not to be obese. To me it feels like when republican parents were crying about Michelle Obama trying to get kids to eat healthy "waaaa what if I want (my kids) to eat unhealthy, you're oppressing me by showing me better ways and by showing me the concequences to my actions!!!!" (Side note, yes I know Obama's program wasn't perfect but that type of argument against it was utterly stupid)
Edit: here's the blog that "dubunked" fatness being bad
http://bigfatscience.tumblr.com