There are legitimate security concerns in having large numbers of your citizens using a platform that directly controls algorithms to show specific content, run by a foreign company known to have direct government control, force them to do what they want. TikTok may deny it, but Chinese law allows the government to essentially force them to do whatever they want. And yes, that includes the US and other countries as well, this is not a US or China thing, it's a general foreign narrative control issue.
Nothing prevents the Chinese government from forcing TikTok to adjust their algorithm to promote whatever points of view they want and suppress others. Many other countries do the same with their own companies, whether overtly or not, but a foreign government that is known to do what they want, regardless of any agreements, treaties, etc. having the ability to directly control what your citizens see is a massive security risk.
Is a full ban the right approach? The US doesn't really have any way to force TikTok to ignore Chinese government demands, it is a Chinese company after all. The only way to force compliance, is not allowing them to operate in the US as the alternative, and the company can choose to either do what the US government wants, or abandon the market. It seems that's the stage the politicians have decided we're at, and TikTok hasn't made changes to accommodate US requirements to operate here anymore.
It seems to me that simply exposing the possibility of Chinese interference to the public should be sufficient. Once the public is aware, then every individual's personal, informed decision to use or not use TikTok is a protected form of political expression.
I expect that most Americans who use TikTok would choose to keep using it, but that's an option that freedom of expression gives them. They have a right to consume Chinese propaganda if that's what they want, in the same way that they have a right to listen to any speaker or read any book that they want to.
And thus we get to the fundamental issue behind all of the bullshit. Where do we draw the line to try and balance individual freedom and national security?
Trump has clearly been a Russian asset since at least the 1980s, yet the American people have elected him President. Twice. You trust the general public to make a determination about social media apps intentionally infusing themselves into the culture and are being leveraged by their government to undermine our country from the inside by managing content and spread?
I've worked with the public for 20+ years daily in various customer-facing jobs, and I barely trust the average person to just continue breathing, and that's an involuntary body function requiring no thought. I definitely don't trust the average person to be capable of an educated analysis of their content consumption and possible negative influence. Not that I trust the government much either, but at least there's some people in the machine that actually know what the fuck they're doing and talking about to try and steer the ship in the right direction.