.... is it just general asshattery?....
As far as I can tell, yes. Besides not liking their chest bumps and maybe a few here and there not caring for their brand of tennis, i've never heard anything at all substantive about them. Oh, that and they're American and some people are too ignorant to be able to get past their "frat boy" exuberance to see how much they actually do for the sport (the commentators postulated that they spend more time and energy trying to help the sport than anyone on the doubles OR singles tour
) and how hard they work and how good they are.