Because......well some people need it spelled out in black and white. Let's see where we start......have you noticed how very few 747-400s are operated by US airlines? It was the US airlines that really pioneered twin ops across large expanses of water. Suddenly it became perfectly acceptable for twins to fly ETOPs all over.....but wait!......ETOPs started 'disadvantaging' US airlines against foreigners flying all those lovely big 747s into the States! The FAA, ever willing to look after US airlines interests (as all the aviation authorities obviously are keen to look after national interests) therefore has a vested interest that 747s now (remember Boeing are hardly selling any now, and none to US operators) have no advantage over 777s (which everybody wants) and 767s in over water ops. So, in the case of a single, uncomplicated engine failure, it is to US airlines interests if every jet, twin and 4 engined, has to obey the same silly rule- silly for a 747, but dead sensible for a 777 or 767, of scuttling for the nearest runway!
I'm still amazed at their nerve trying to fine BA for continuing with one engine out on a 747 when they have accepted that the United 777 can fly on one for 3 hours and 8 minutes to a diversion airfield. When politics sticks its nose into safety issues, we should then be concerned. Meanwhile, we know what is safe in an aeroplane that has this sort of redundancy built in, and the flight continuation policy remains unchanged, despite thunder from across the Atlantic that we are trying to gain 'an unfair advantage' because foreign airlines operate a aeroplane with more redundancy!