I use them. First of all, they're still absolute dogshit (but Apple promises good changes in next OS?). And the choice is to either allow social media in its current, extremely toxic form, or ban it entirely. There's no "Allow Instagram but only let them see the content their actual friends post." No "Allow TikTok but let me allow-list the influencers that show up in For You." The peer pressure on kids to be on those platforms is intense though, and many parents seem to be allowing it.
I think the theory behind the law is that experimenting on children to optimize (without any ceiling) for increasing watch hours, which 100% of the social media sites are doing at all times and have been for over a decade, is unethical and should be illegal.
If adults want to subject themselves to that, it's their business, but for children, we restrict all kinds of things from even being marketed towards children. Things (especially addictive ones) that are very obviously bad for humans, like cigarettes and liquor. Why? Because otherwise there's a huge incentive for the makers of those things to do so, because of how naïve kids are.
A wildcard here is how if at all to address careless, negligent parents of course. For the things so obviously harmful like drugs, it becomes criminal to give those to your kids. But perhaps social media falls into about the same category as R-rated movies, where we just say "it's bad parenting to take a 6-year-old to one, but not illegal." Another area where we have 'censored' things for children for decades - without any kind of scary police-state regime materializing and forcing biometrics and recordkeeping at the box office.