There’s no expectation of privacy in a public place which includes public roads.

The US doesn't have any general restrictions against filming in public.

Court rulings citing the amendments above were made to affirm that law enforcement are not an exception to the general rule that filming is allowed.

US doesn't have an expectation of privacy in public spaces. This allows for filming of anyone/anything in a public space. Following a specific individual or vehicle in public doesn't make a difference.

Simply filming in public is not a nuisance and should not be illegal.

Heard of ALPR networks?

It's seemingly creepy, yes.

It might warrant a restraining order, sure.

Outside of judge's opinion that it constitutes stalking or threatening behavior, what exactly is the crime here?

Perhaps it's a neighbour who keeps the same hours and works in the same city block, etc.

Perhaps it's a PI hired to follow you because <whatever> .. is that a crime?