Well, firstly, don't connect your machine to networks you don't trust the members of :)
If your machine's manufacturer still supports the device, check if they have any firmware updates available. Hopefully they will have recent updates that include a fix for the AMT authn issue.
That’s an interesting question, isn’t it? Even more, how AMT was enabled in the first place, if the UEFI has no option for it.
And I’ve had massive issues with AMT before – for some reason, on Linux, the ME would force a reset of the network connection every 90 seconds (which is why I use an ancient realtek network card currently).
Possible explanations include bad defaults in the UEFI, a store sending me a used part instead of a new part, etc. If we go into conspiracy territory, NSA TAO interception would also be on the table. Very unlikely, though.
If your machine's manufacturer still supports the device, check if they have any firmware updates available. Hopefully they will have recent updates that include a fix for the AMT authn issue.
If you want to disable it, Intel has provided a mitigation guide which has instructions on disabling LMS (which AMT is part of): https://downloadmirror.intel.com/26754/eng/Intel-SA-00075%20.... I've not had to follow it myself, good luck if you do :)
I'm just repeating stuff I've read from MJG, take a look at his FAQ around this issue: https://mjg59.dreamwidth.org/48429.html