Things have to be put into context.
Mayweather used an IV infusion of saline solution (saltwater) and vitamins after the weigh-in and also after USADA collection agents had arrived to take samples for testing. IV infusion's are neither inherently prohibited, nor are they illegal. The main reasoning for prohibiting IV usage over 100ml per 12 hours is that it can be used to mask PED usage. In Mayweather's case, USADA already took samples before he used the IV. If Mayweather's intention was to use the IV to mask PED usage, then it was pointless. If the collection agents had arrived after the IV was used, then undoubtedly the TUE would have been rejected and Mayweather would have been sanctioned.
In comparison, Pacquiao also attempted to obtain a TUE in fight week. His team requested a TUE to allow a painkiller injection on fight night for his shoulder. This was rejected as the "injury" to his shoulder had not previously been disclosed to USADA or NSAC.
The likes of USADA and VADA aren't perfect, but the idea that they would be a party to giving one athlete and advantage over another is beyond ridiculous. Catching either Mayweather or Pacquiao or both using PEDs would have been a dream scenario for USADA.