If the default wasn't "almost nothing" then you'd be sanctioning exposing some kids to content their parents didn't want them to see. If there's no economic incentive to tag content then it's not valuable content for kids.

Ultimately the problem is the provider knows what category the content is and the parent knows what the content policy is. Providers can't say whether it's "safe" or "unsafe", only what standards it complies with. Some parents will have weird policies like "only G-rated movies or any Jim Carey movie" that can't even be delegated in any reasonable way to providers.

So the header has "PG-13, US-legal" because it's a movie rated PG-13 and constitutionally-protected legal content in the US, and whatever other markets you want to open up. Providers could even include AI ratings so as to mass-tag their content at low cost, and parents can decide if a particular AI rating is okay.

Parental controls could even restrict official ratings to country of origin, so if you approve PG-13 it'll block that content from countries where you can't sue them for lying about it.