I guess with tech companies - "operate in" is a little unclear. Someone in China can visit your website (assuming it's not blocked by TGF) - and it might be very difficult for you to know (they could be using some Proxy or VPN).
In Apple's case - they literally have stores selling phones in China. AFAIK, that's how most people get their iPhones in China.
In the US - you have to pass all kind of FTC obstacles to sell a device. In China & other countries - it's the same way.
It seems like an overstep for the US to say - "We get to say what devices are available in every country." It also seems like an overstep for the US to say China can't have a Great Firewall and Censorship even if that's extremely "un-American". Us having free speech is extremely "un-CCP" - but for the time being - we've still got it [=
More abstractly, corporations are artificial legal entities that are created by governments in the first place. So why can't governments restrict what those artificial entities can and cannot do?
I'd say that depends on whether the subsidiary in China is financially independent from the mothership. If Apple in US profits from censorship in China or subsidizes it, it's not unreasonable for US to interfere with that.