They can spend because they earn it -- you can spend a billion pounds if your earnings are more than that. But it's true that FFP only maintains the status quo, preventing another Chelsea or Man City. If they honestly want to improve competitiveness, a wage cap is the way to go.
With the players they've been concretely linked with right now (Depay and Gundogan, if I'm not missing out on someone), I'm not worried either. But when you have so much shit to fling, some is bound to stick on the right spot every now and then
It's an obscene amount but FFP doesn't come in to it for United; they're just spending what they make. Their £750 million, 10 year deal with Adidas kicks in next month, which will be the icing on an already doughy cake.