If Federal law truly supercedes state law as hams claim they do, then
how are local state laws prohibiting using scanners to monitor the
police legal???

The federal ECPA law clearly says that monitoring police transmissions
is specifically allowed.

From anywhere in the U.S.

And "anywhere" would inclide from a vehicle.

And if I recall correctly, the ECPA even goes so far as to say that
monitoring of police may not be prohibited.