They decide what content is viewed by millions of teens who will be voting in a few years. It’s not that far fetched to show them more violent content / mildly biased content / etc in order to move their political views a bit.
Perhaps an appropriate response here would be to detect this manipulation and highlight and educate people about it. Possibly create regulation to outlaw specific types of highly problematic aspects of it. But I find it hard to simultaneously accept that it's both so subtle as to be undetectable and unregulatable and yet so terrible it warrants arbitrary restraint of free speech.
Protecting free speech of geopolitical competitors (China) and their economic agents (TikTok) is legitimate avenue for democratic destabilization.
[1] https://www.bloomberg.com/news/features/2018-10-04/the-big-h...