Totally agree with Iccz, Mayweather has never been a real risk taker and a fight with Pacquio right now would be the biggest of his career, he just doesn't fancy it so came up with an excuse as only he knows how.
Funny how nobody else seems concerned(in that regard at least) about fighting Pacquio
As already suggested, it's mind games, he wants to put Pac off the fight and that's what he's doing for as long as he can. I bet if Pacquio hit a slow & steady decline Mayweather would be much more interested, and the exclusive drug test demand would suddenly be magically resolved.
I think this is a fight that could happen, but only if the risk factor largely drops within the next 2 or 3 years(ie. Pacman slows down).