Why did Mi lock the bootloader for Mi devices?
Mi is an international brand now, but it's only in few countries currently, and is still in the stage of expanding its international market. So it's difficult for users from countries where Mi is not officially sold to buy Mi phones.
Many users have taken the risk of buying from a unofficial reseller. But they sometimes get fake Mi phones, or Mi phones with unofficial ROM loaded with bloatware. You guys must have seen some users asking in the forum whether their Mi phones are genuine, or that they have weird ROM version installed on their new phones. This is really bad for user experience, and it might even cause property loss if some malware are installed. This situation also exists in China market.