Without it, ISPs can stop offering internet access and start offering YouTube access. Here's an example of a plan in a country with weak net neutrality.
Just to be 100% clear with the packages shown, they offer the benefit that those apps don't count towards your mobile data, rather than just being available.
Still not great though, since it just increases the moat for existing companies. Also companies in the states have tried exactly the same thing before with their video streaming service not counting towards you data cap.
https://en.wikipedia.org/wiki/File:Screenshot-2017-10-28_MEO...
If you believe some other law already prohibits this, please tell me.