Domestic governments shouldn't let hostile foreign governments the ability to exert soft power over 1/2 of their population. Hence why China banned all USA based tech companies from operating there.
It seems there are indeed things that can override citizen's free choice even in the "lighthouse of democracy and freedom", and CCP didn't make a mistake for building the firewall. My need to use Shadowsocks to use Google instead of Baidu or some other crap was simply a collateral damage.
Of course, the Chinese censorship is way more intensive, but this act makes a dangerous precedent.
tiktok.com links were available in China.
Obviously the USA doesn't have a GFW, so they can't actually block tiktok, just ban it from the app store and prevent business from resolving in the US around it (e.g. paying content creators).