The interesting discussion to be had here is about what good policy is, not to say "A country can make whatever laws it likes". Well duh.
We all have an interest in a functional global system. To that end, it is beneficial of countries at times defer to the laws of other nations (https://en.wikipedia.org/wiki/Comity). Maybe this is such a case. Possibly we would prefer not to have other countries try to interfere with mergers happening without our own jurisdiction too often.
As an American citizen, would you want to restrict regulators from being able to block the business of a company that merged with a CCP controlled company in China?
On what grounds would that business be blocked?
On grounds having being state controlled? I'm not sure US regulators do have that power. On some pretend grounds, to be able to wield it as a geopolitical weapon? Probably not.
On some narrow national security reasons? Maybe.
Any grounds pertinent to American interests, frankly.
overall, the Commerce Clause in article I of the US constitution grants the federal government power to regulate commerce with foreign nations, which includes import / export restrictions. For example you can't really buy things from Iran or Cuba without major restrictions if at all.
if a trade embargo doesn't fall under the realm of "geopolitical weapon" I don't know what does.
My overall point is that details matter for good policy. A regulator having the power to ban a foreign merger for supposed anti-competitive reasons, but really political ones, is not the same thing as, say, a legislative body passing sanctions legislation.
In other words, just because the UK wants to ability to sanction a Chinese business does not mean the Competition and Markets Authority needs to granted the ability to interfere with Chinese mergers.
How much more geopolitical do you have to get? You've been trying to starve an entire country for more than half a century because a bunch of your citizens are pissed off they got kicked out of there 30+ years ago and your politicians need them to win vital nation-wide elections.
Yes, I know about all the Cuban abuses and all that other garbage, but that doesn't make it right because the US is allied to <<soooo>> many other human rights violating countries and it doesn't even bat an eye.
If the US wants their regulators' decisions to carry weight in another country, they can pay for the privilege via trade agreements, same as it ever was.
Companies are a tool that should serve the public, if they cease to serve the public, they should be bought in line, destroyed, or (as is threatened in this case) exiled. A democracy is meant to be for the people, not for profit. Profit is just a motive to try and incentivise things that are valuable for society, when that isn't achieved, regulators should step in to fix it.
Companies that are too large have too much money, and too much power. They are inherently anti-consumer and cause harm to the very fabric of democracy. In any sane world, we would be breaking up a huge number of these large corporations into much smaller pieces.
The electorate has a responsibility to protect democracy and not let companies break the law of the land, they have no responsibility at all to companies.