I came across this last night on ba.com. The requirement was news to me, and I assumed they were wanting to double check the card validity in some way. Of course, the info they requested and I supplied was incompatible.

After trying a couple of times, I noticed the 'not this time' (or similar) button, clicked on that and got the booking confirmation.

The message said that if I didn't register, I would be allowed two more transactions and then no more.