The difference is the US government believes that TikTok is beholden to the Chinese government. When a corporation acts this way it is an invasion of privacy. When a foreign government acts this way it is espionage.
If TikTok is sold to an entity the US government thinks is sufficiently independent from a foreign government, then they can continue spying on users.
Alternatively, they may be able to registers under the Foreign Agents Registration Act. I don't know how that would impact TikTok's ability to operate though.