So it is a tethering restriction after all?
If the conditions of contract mention something we do not know, it's on us. We have a saying here that translates to "always know what's on with you and what's against you". Their way would not be wrong. If it's not mentioned in the conditions, you can sue them and you should win. Disabling a common service on a phone; i.e. tethering, is not something to neglect by the dealer/ISP. At least if the phone is bought from somewhere else.