The American market is saturated by garbage, so trying to get a foothold here is not the easiest thing to do, but there is a market for music like Opeth. I feel the internet has helped many bands like Opeth gain sales in America, which would otherwise be next to impossible.
America combines a decent sized population, with a high buying power per person. Because of that, Americans spend more on entertainment (movies, music, so on..) then any other country, though maybe not per person, but definitely more overall. A big movie will do almost half its total gross in America alone, and the top selling albums of all time will also do about half their sales in America.
Because of this, there is a lot of interest to get a foothold in America. The problem is the record industries here, who have radio and MTV/VH1 wrapped around their fingers. A band like Opeth has little chance to get the exposure that a boy-band might. Still I know there are still many untapped people in America who would eat up music like Opeth if they were exposed to it.