Actually, they are not "guilty of defying Federal law" on detainments  --  they are just declining to accept the unfunded mandate of keeping arrestees in jail longer than they need to to meet our own State & local law, just to suit the convenience of the Feds.  

There's actually no Federal law that requires them to do so, and quite a few cities don't oblige the Feds with this favor, not out of principle but because their jail expenses are already killing them.

Those extra jail days, BTW, are NOT reimbursed by the Federal government.