Halloween is not Samhain. It does not share its roots with Samhain. All Saint's Day was moved from May to October in the 4th or 5th Century.
As to whether they can celebrate Samhain, that's a different question. But sure. If it floats their boats, they can celebrate Samhain. If, however, they're just tacking "Samhain" onto whatever completely non-Samhain things they have going on at the end of october, that I'd be inclined to call misappropriation.