The government is auditing TikTok, not using TikTok to spy on US citizens.
The US is trying to determine to what extent TikTok is a domestic security threat.
CFIUS is an organ of the US government designed to prevent foreign intrusion that risks national security. That's the entity involved here. They don't touch ordinary US citizens.
This is not domestic spying. It's investigating the capabilities of a foreign power on US soil.