If you assume the OS is coming from a compromised vendor, what's to stop it from making wireless network connections on the sly? Or adding a 'phone home' payloa

If you assume the OS is coming from a compromised vendor, what's to stop it from making wireless network connections on the sly? Or adding a 'phone home' payload to any outgoing data copied to removable media? You could physically destroy the wireless capability. But not using or destroying the media inputs would leave you with a fancy typewriter.