It’s a common meaning of the term, at least according to Wiktionary:
v. (2) (transitive) to make, transport and/or sell an illegal version or copy of a copyrighted product
n. (2) An illegally produced, transported or sold product; contraband
a. (1) illegally produced, transported or sold; pirated
There is are a few other meanings, clearly not relevant here (we’re clearly not talking about actual boots, liquor, or American football), but there is one more relevant meaning:
n. (3) (music) A remix or mashup that is a combination of two songs but that is authorized and audited for copyright use; prevalent in the electronic music scene
but we’d call that an “official” release here, since it’s sanctioned by the artist or label.
BTW: Maybe we should split up “bootleg” into multiple categories, but at some point it seems we’d just be making moral judgements—I suspect there is a large agreement that e.g., posting a clip of a concert you went to on YouTube is OK, or that trading of live recording where the band was OK with it isn’t a problem either. And that attempting to claim someone else’s music as your own and sell it on iTunes is not OK. Maybe those should be split.
But ultimately MB is supposed to be an archive of music metadata; largely, I think, we deal in facts, without expressing an opinion on them. We currently use “bootleg” to express a fact, that a release was not authorized by the artist or label. But if that’s OK or not is an opinion which, once you’re out of a few easy cases, many people will disagree on. That’s not something the normal editing process would work for at all. (It is something you could use tags for, though.)
Listing a release does not mean we endorse it. It merely means we think it exists (or existed).