I suspect that the reason for the ban is that "fractional ownership" arrangements amount to public transport operations under UK law (Air Navigation Order). Public transport operations by non G reg (or EU registered aircraft on intra EC routes) to or from the UK need a permit from Dept of Transport. I understand that US legislation does not require fractional ownership companies to have an AOC, which is necessary to obtain a permit from DOT. It appears that the only way "fractional ownership" operators can operate in the UK is under an AOC but I can see no reason why it has to be a UK AOC (unless their main business is cabotage).
I agree that traditional corporate operations are a different kettle of fish.