Probably because you can just modify the OS to say you're old enough. With a closed source OS you can't.

In reality it just means you can't use the OS as age verification, and will have to use other means if you're into that. Personally I just slap on a VPN and go somewhere that hasn't succumbed to this idiocy...

I'd imagine most kids using Linux are either being actively taught computers by a parent who's paying close attention, or so smart that nobody is stopping them from doing anything.

The logic is none of these distros would do shit to comply with this, and enforcement is almost impossible. This is just saving face.

I don't know what the Cali gov was thinking

But it makes sense to me, based on the commercial incentive. It's not that FreeBSD doesn't form the basis of products that will need to comply with the law (whatever Apple calls their desktop OS for instance), but FreeBSD itself is only for the leet.

No, you don't have to be a genius to use it, but you do need to be very weird, in that you want to use your own actual brain to solve problems, and you want to fully understand what the thing is doing, from bootup to shutdown. If that's you, you're a very odd person, according to many.