I think it makes sense. Xiaomi had a lot of issues with shady companies importing cheap phones for the Asian market, flashing the xiaomi.eu ROM onto them, and passing the off as official.

The fact they still allow you to unlock the bootloader in the first place shows they value the freedom to unlock these phones. When they came up with a plan to combat these reflashed phones, they could’ve simply locked down the bootloader entirely. Setting up a whole web service with SIM validation and such isn’t cheap, they spent money on keeping their phones open!