The existence of Sky Marshalls didn't prevent the hi-jacking of 4 US aircraft on 9/11 did it? That was proof enough that they don't work as a deterent. The fact that (to my knowledge) only US and Israeli ac have Sky Marshalls suggest this is just another way for the Administration to keep foreign ac out of US airspace and protect (financially) their floundering airlines, as other nations do not have enough marshalls required to operate into the US.
I echo the sentiment that we should ban ac with Sky Marshalls on board from UK airspace. Full screening and pax profiling is the only way to move security forwards.