It's not about deciding what's acceptable for one's kids, it's about limiting the range of programming so that one can choose if that range is acceptable. This is a difference from traditional channels which have a distinct flavour and are subject to watchdogs (in my country, UK, at least).

No parent has time to watch TV with their child all the time up until they're 18. You might say 'well don't let them watch TV' and that's a valid choice. But other parents want to choose safe ranges of programming and have someone else police stuff to make sure it stays in that range.