Assuming at least some of the Sheriffs actually mean what they're saying...
Sheriffs deputies block/arrest US Marshalls
How do the locals know that Feds are there, or who is being arrested, where and when? Are they notified of Fed activities in their AO, and if so, is that legally required or just professional courtesy? Could the Feds just grab their target out of the clear blue, and whisk them out of state?