And why should you need permission to do this?

Xiaomi historically had a problem with resellers installing malware in custom ROM on their phones, so they started putting up more and more obstacles to unlocking the bootloader over time, while still providing an avenue for legitimate customers to unlock.

I don't know what spurred the current action though.

It's worse than that. All this China big brother talk is just a variant of xenophobia. It's a talking point they've been trained to slam their foreign "enemies" about without ever thinking about it at all of what the actual harm they're concerned about would look like.

Name one modern phone >= the size of the legendary Sony Z Ultra.

It always blows my mind some people actually found Apple's defense convincing.

The iPhones didn't inform users when they were throttling because they had an old battery. Apple kept the throttling a secret and coincidentally it helped them upsell new phones to people with old phones. This type of functionality was also unique to Apple, it's not like this is the only choice they had and an industry practice.