DeletedUser443 Not being smart, no, i actually can't think of any reasons why they would benefit from tracking who unlocked a phone. That's why I asked :)
First of all, i don't know if they actually have the ability to track if someone unlocks the bootloader, maybe the requirement to have an internet connection is solely to check if the corresponding device is being sold with the option of an unlocked bootloader, and stops there.
But to answer the question why they could benefit if they actually track;
One reason could be just for data analysis, to keep track of what percentage of the phones sold use this 'possibility'. If that number is significant, maybe it is keeping them providing that 'possibility' as perhaps a fair amount of their sales comes from that 'possibility'.
Another reason could be, In some cases IMEI is linked to an identity, depending on how u bought the device. Combining this information with other possible data already tied to your identity, could provide a much beter profile to sell to advertisers.