Whoever you blame for it, it is a horrible thing to have happened, and it certainly won't do anything to improve the relationship between beekeepers and their non-beekeeping neighbours.
I'm sure there's more to it than simply "nasty bees attack and kill dog". I wonder what made so many attack? But I suppose that's really not the point here. If we're not careful we'll end up with the situation that we won't be able to keep bees in built up areas or close to neighbours property (or on allotments as these bees were), which would be a real shame because 99.99% of the time the bees will not cause a problem. We as beekeepers really don't want our hobby to be perceived as a danger to children, small animals and the general public!
The only solution is education. If you see a beehive, DO NOT APPROACH IT, unless you are suitable trained and wearing the right gear!
Would warning signs help?
But I think the point has been made that if you put a warning sign up then you are admitting that your bees may be a danger to others.