That sounds the most likely meaning, but it assumes that the reader is aware that bison is not, eg, fatty - but how many people eat bison to know this? In short then, it's not at all clear.
Just to add, we often say 'guilt-free' when an activity isn't destructive to the environment ('guilt-free holidays at home') or does not involve the exploitation of workers in poorer countries ('guilt-free shopping') or of animals ('guilt-free cosmetics'), etc. - basically, all those things that the middle-classes in the West sometimes feel guilty about!
:up: Yes - that's why some people eat lamb but not beef; because sheep can be fattened on land that can't be productively used for much else - whereas cows are raised on land that could produce much more food value if it were used for growing fruit and vegetables (to say nothing of the amount of arable land needed to produce food for the cattle). I don't know how this fits in with bison though.