Exactly this - a foul ball is always a strike but you can't strike out on one.

Also, if you bunt on strike 3, and it's foul, you're out (and it's considered a strike out IIRC). This is because, way back in the early days of baseball, there were guys who would just stay at the plate for a really long time bunting balls foul and wearing out pitchers, and everyone hated it. So they changed the rule.