> why would law enforcement and courts go along with it?

They don't want to deal with it. If someone has one of those SovCit license plates, you know pulling them over is guaranteed to result in frustrating verbal sparring match, which may result in the cop giving up (which is then shared as evidence for the SovCit movement's effectiveness!), or may escalate to a physical altercation.

Whether or not the SovCit practitioners understand that's what's happening is anyone's guess.