because like every race, nationality, religion or whatever - the are complete cocks and there are people who do things like this (obviously), but maybe just maybe not every arab guy is the same and maybe this source thought that blowing up and killing people is wrong - a nice arab guy, now that would be a shock to the western world :S
but its true we don't truely know the reasons for the attack (yet) - al queda could have just thought 'we will claim it even if we didn't do it'. But they have been the biggest terrorist threat of recent years, hence we instantly look towards them. 15 years ago we would have instantly thought it was the IRA.