This is part of a test to see how much nickel-and-diming US can get away with. Perhaps they are trying to subsidize the cost of lost baggage. As I understand it, skycaps do not work for US and (like waiters) get most of their income from tips. Presumably, the signs are there to make sure that travelers know that the $2 goes to US rather than the skycap.

At PHL, where they are trying this same plan, I have seen skycaps putting little stickers for each bag on a sheet. I would assume this is how US tracks the revenue from skycaps. The skycap who checked our bags told me his volume is down considerably since US started to charge $2.