Exactly I completely agree with you. I'll try find the terms of the 12 month trial but it doesn't make sense as it works fine on my other devices and they told me it should only work on one device..
I don't know where they are getting that whole one device thing from, but it's the first I've ever heard of it. To be honest I think the real answer is that they haven't implemented it properly on the ATV4. Hopefully they fix it in future with an update, but at this point I don't think the so called 'specialists' will be able to help.