They could create a standard and then let individual countries tell vendors "hey, you must comply with B.123 in order to sell in our country" -- but they can do that anyways. If a government wants snooping capabilities, you can bet every vendor will add it to get their business. It's still the government that decides if it's mandatory to turn on or not.
Again, I'd like to hear the full path from the ITU taking a vote, to my ISP suddenly snooping in on stuff. I can't figure it out.
My sport (paragliding) has been destroyed by similar actions from the governing body in the last 18 months. They don't need to say "you should do X". All they need to say is "we think that maybe you should do X" and suddenly everybody falls into line and does X. It's not about enforcement, it's about not being seen to contradict a perceived authority.
ITU votes to allow some kind of snooping in the standards.
Government asks ISPS etc to follow the internationally agreed standard. "We'll only use the snooping stuff for terrorists and images of child sexual abuse, really."
Government uses this new compliance to the standards to get your ISP to snoop on stuff.
The governments take this circuitous route so that they as individual governments don't get attacked by local libertarians. Defeating a measure like this in one country is hard; defeating it across international treaties is very hard.