Apparently there are multiple reasons why the uncut version CANnot be released in the US.

(1) yeah, you had the case with the mom renting the dvd for her kids, just to get a big surprise a few hours later. The thing is, it wasn't her complaining per se that got the dvd pulled.

(2) it was Adrienne Barbeau's contract. Her contract states that no nudity of her can be played in the US (apparently she doesn't care if her Bar.. well, you know, show up overseas).

(3) also, the cut PG version is Craven's preferred version. Even so, it still comes down to MGM US not having the rights/permission to play SWAMP THING with the extra bits.

Which is why the uncut version can play on MGM-HD UK.