I'm not sure it is legal, but as a practical matter, so long as TradeSports doesn't have assets in the US, it's hard to enforce any judgment against them (I don't think Scotland would enforce a US money judgment against TradeSports.) I have, however, heard of Federal or state authorities going after credit card companies based in America, for facilitating illegal gambling in online casinos.
So I imagine the fuzz could shut down betting from the US, if they put forth the effort.