This does seem very strange. What if you haven't earned any avios in the relevant period other than from your BMI credit card? Would the avios expire as soon as you transferred them in? Presumably there is some intercompany accounting between BMI/avios/BA which means BA is receiving a benefit for accepting the transfer.

You might consider whether you can derive any benefit from the The Unfair Terms in Consumer Contracts Regulations 1999. Not saying that you can, but that is where I would look next.