Far as I'm concerned, any commercial product that contains swearing has nearly always been Rated M For Mature, which is a more verbose way to say 18+ only. Rated T for Teenagers and Rated E for Everyone straight up do not have swearing, period.

Amateur products have been the exception to this norm, because let's face it: The commons don't give a shit.

But platforms/publishers themselves are not the commons, they are businesses and they have legal liabilities: They do give a shit.

So while Youtube is asinine, flagging a video that contains swearing as Rated M (aka 18+) is reasonable. The only thing unreasonable is inconsistent enforcement. If something contains swearing, it's going in the 18+ Only bin.