It's dumb, but Wickard v. Filburn makes basically anything involving physical goods "commerce". I'm sure there's a ruling somewhere that says something like: people entering the country subtly alter the restaurant market (not really any dumber than the Wickard v. Filburn rationale), and therefore the feds have a right to search everything.

I think it would be a lot harder to do that with speech though. Maybe you could argue that the telegraph line itself impacts international copper markets or something, but there are non-tangible based communication methods.