The US owns and regulates the frequencies TV and radio are broadcast on.
The US has also historically regulated who owns media companies.
As for RT vs TikTok - good question. My guess is that scale and influence have a lot to do with why regulating TikTok was prioritized. Also RT has been removed from most broadcasters and App Stores in the US.
I’m mostly just guessing because I’m at work right now, and while I’m on a pee break I don’t have the time or energy to research the nuance of foreign media ownership legislation and regulation.