It seems difficult for the company to ensure that you have a work permit for any country you might be going to. There are countries where you really don't want

It seems difficult for the company to ensure that you have a work permit for any country you might be going to. There are countries where you really don't want to get caught working without a work permit, but even in more lenient places in general it's just dangerous for the company to be caught in this situation. And to legalise that work (even between EU countries) means they have to get involved with yet another fiscal system which is probably a lot of overhead for just one guy wanting to work in any country he likes.