i think a large part of this depends on the quality of the SERPs. if i search for "red widgets" and google serves up a page full of sites that don't apply, then of course i'm going to want to block them. but that's not the sites' fault. it's google's fault for messing up the SERPs.
if those sites then get penalised for that specific keyword, then okay. there is no damage done. because they didn't want to rank for "red widgets" anyway. but if google takes account of the penalty for other keywords too, as if it's some kind of sitewide demotion, then i don't think that is fair.