A. Doing what harm? People just throw this around and there's been no evidence except, "lol it's a social media company".
B. It's not within their authority unless there's a specific national security problem. So what about TikTok is going to breach national security? Are they stealing military secrets? (They were already banned from government devices along with other social media apps so the answer is no. They're not.)
The Constitution is supposed to protect us from the government just pointing at us and declaring us criminals. Today it's TikTok tomorrow it's you.