They don't, and cars can still come in to Swedish ports... There's just not anyone there that's willing to unload them before the ship has to leave again.

The unions are allowed to strike because Tesla sign the collective agreement that regulates the use of strikes as well as guarantee a minimal wage, pensions and so on.

Tesla can sign this agreement any time they want to, but they're choosing this fight while the unions wanted regulation.

It's on them, not anyone else - and Musk is perfectly free to take his business elsewhere if he don't want to comply with Swedish customs.