same thing as far as stratasys are concerned. And they could argue that by using unapproved filament you're still invalidating the warranty.
Which is fine, as it's your call.
But there is no way it should be prosecutable offence.

If you choose to 'break' your machine with dodgy filament - that's your choice. You can't stop stratasys - or anyone else, this is a test case after all - turning round and saying they won't fix it because you weren't using their aproved materials.
They have the right to do that. But it would be insane to make it illegal.