Written with a slightly more precise wording, they know Linux users have full control of their devices, so cannot keep them from using it in ways the company does not like. In this case, fine. Go away and take someone else’s money.

Stated from another angle, they won’t support it because they can’t hijack it for their own purposes.

This. This, this, this, this!

My linux computers are rooted. I can get root any time i need it and nobody is refusing to offer their sevices on linux because it is vulnerable.

Nobody ever points out that when any app wants root, you get a dialog to ask if it can have it. If you don’t know why it’s asking, say no. It ain’t rocket science.

Now, if you are going through customs and you don’t want them to copy your phone and read all your personal documents, that is a different situation. Lock your bootloader unrooted and encryped to the nines. Preferably use a phone with almost nothing on it.