Not everyone does it, it's just a method some people use to promote more obscure picks. I only downrate obvious songs if they're only tangentially thematic; if someone picked, say, Judas Priest's Burnin' Up for a fire theme just because it has "Burn" in the title, I'd downrate that in addition to downrating for being off-topic,, but I won't penalize Deep Purple's Burn. If you pick demo stuff there will often be one or two players that may downrate it just because they don't like lo-fi recordings, so there's not really a guaranteed method of getting a song to win. Personally, if someone submits an obscure song that's both thematic and really good, I'll uprate it extra just for showing me something new.
As for what's obvious, that partly comes down to the tastes of the forum. Demolition Hammer, Immolation, etc certainly aren't mainstream, but they're pretty revered in online metal circles, so that's why you'll get some downrating there. Pestilence last round is probably a more popular band than either of the aforementioned, but the song was a less literal and obvious interpretation which is why I think people were more forgiving.
So it's really just a crapshoot until you've figured out the forum's collective tastes.