Stalking is very illegal in most US jurisdictions.

Maybe if you are threatening someone. But if you are just following someone in public, that is okay.

Stalking is not "just following someone".

From Stalking Prevention, Awareness, & Resource Center (SPARC): " A pattern of behavior directed at a specific person that would cause a reasonable person to fear for the person’s safety or the safety of others; or suffer substantial emotional distress."

https://www.stalkingawareness.org/definition-faqs/