That's a separate issue I don't support outside of situations where they can reasonably prove/show you crossed without alrady going through the search. Luckily, at least so far afaik, it hasn't really been deployed in the maximalist way that turns the whole country into that zone. It's a lot easier to justify near physical land borders like Canada and Mexico than it is if it's being used within 100 miles of very controlled impermiable borders like international airports. Has it been used outside of the land borders?

Well intentioned laws today can be used by illintentioned actors in the future, and mild legal exceptions can become significant ones with a couple additions.

The 100 mile zone is a good example - that 100 mile zone assumes the entirety of the great lakes is international waters, even though Lake Michigan is 100% American Territory and over 300 miles long. So, you could be in a landlocked county of Indiana, and be harassed by the federal government for being in a border zone.

Whats to stop someone from saying, the entire Mississippi river and its tributaries are a border zone? With that you could get 80% of the US population into the border zone.

By enabling some "rule based" decisions that skirt the constitution, you enable a legal mechanism for constitutional rights to be violated without any legislation.