why can they just sue someone for refusing to buy something

From the article:

As part of the agreements, the Chinese fund granted the remaining minority shareholders an option to require it to purchase their remaining shares for about $9.5 million, now valued at roughly $11 million, by early December 2025.

especially when buying it would be illegal

Legality in country A is completely irrelevant for suing someone in country B.

Also, in many countries having a realistic chance of winning the case is not a prerequisite to suing anyone for anything.

ruling that Air Canada had to pay Moffatt $812.02 (£642.64) in damages and tribunal fees

That is a tiny fraction of a rounding error for a company that size. And it doesn’t come anywhere near being just compensation for the stress and loss of time it likely caused.

There should be some kind of general punitive “you tried to screw over a customer or the general public” fee defined as a fraction of the companies’ revenue. Could be waived for small companies if the resulting sum is too small to be worth the administrative overhead.

If you tell people you are in an open/poly relationship, but your partner doesn’t know about that, you are not in an open/poly relationship; you are just a lying, cheating asshole.

Real poly relationships have a bad reputation mostly because of people like that.