It’s the salt mostly. Especially with indoor cats who often don’t get wet food and so are on the under hydrated side of things as they tend to not drink as much as they should.
Technically any without garlic or onion is safe for them to have a tiny bit as a treat but it’s so incredibly easy to overdo that it’s just safer to not give it to them.
Too much nitrates or nitrites is bad for cats yes, but they’d have to eat an excessive amount for it to be a concern and again the excess salt would be a bigger issue first.
Not typically. You’ll see police along the major highways for speeders and the like but no state border patrol like that. Legally often transporting across state lines is a crime in and of itself but it’s one of those things where they look the other way unless they catch you using whatever item.
Often this is done for practical purposes, because if it’s legal in the state you started in, and might be legal in your final destination, they’d piss off more people that not of they stopped and confiscated from everyone.