Yes, USSF should force MLS to pay the fee
by fontoknow (2018-01-09 12:10:56)

In reply to: Aren't solidarity fees for developing U-18 players?  posted by turtle17


USSF is the governing body in the United States and is the US's representative federation in FIFA. It has a responsibility to enforce FIFA transfer rules.

I see nothing inherently illegal with solidarity fees.