Not even sure it's subconscious racism, maybe though. Eminem is on a pedestal because he is a superstar, and for some reason people expect him to live a more interesting life than they do. People don't want to hear about McDonalds or K-mart because coming from the biggest star on Earth those topics seem underwhelming, even if its just at surface value.. . which kind of makes it even more hilarious that he brings them up.
Or maybe its just musicians in general are not supposed to be that mundane. To the average listener it's really not about the execution, intent, or how well its written. It's about a hot beat and a catchy hook. They don't notice the tight rhymes fly by, but are able to catch "McDonalds" because it seems out of place with their established views on an artist's subject matter.
Actually, the more I think about it, the more I think you might be onto something. I don't think you've hit the nail on the head yet, or even close, but you're atleast holding a hammer. Rap music has always been portrayed as black music. A lot of guns, killing, drugs.... black people stuff. OK, all is well. Then you have this white dude come in, and its weird. But we accept him because this white dude is rapping about a lot of guns, killing, drugs... black people stuff. OK, we are pissed this guy is here, but whatever. Shit hits the fan though when this white dude stops rapping about black people stuff and mentions K-mart, hot-dogs, etc... It goes against everything we've learned thus far about hip hop, and because of years and years of being fed the same images about the music and culture, we react differently to it.
Especially today, when its so much easier to legitimately dislike Eminem, people don't need a good reason to shit on a song. They hear "K-mart", and start typing "Feminem fell off !@11!". Everyone else is saying basically the same thing, so they jump on that bandwagon without having a reason as to why referencing K mart is bad because: a) There isn't a good reason. and b) Other people will provide legitimate reasons as to why the song is bad for them.
Anyways I need to go to bed.