They would be within their rights to do that. But then they would have to compete with other participants in the discussion. On TikTok they can ensure there is no such competition.

The magnitude of the attack is not comparable. One thing is being a bad actor in a network owned by someone else where you can get monitored, caught and banned. Versus owning the network completely and amplifying messages with ease at scale. The effort needed and effectiveness of the attack is extremely different.

Domestic based social media platforms can be pressured to comply with demands such as the DOJ's investigation into Russia's 2016 disinformation campaign on Facebook. Likewise social media platforms based in a foreign adversary would be pressured to comply with demands of that foreign adversary.