It's a manipulation. He likely is not a friend without strings but one that sees people, and therefore, friends as tools to get what he wants. In this case, make money off of them.

By sending you a card, it's a manipulation or an attempt at that to do exactly what you seem to be talking about, being confused, feel guilty, sad, etc. Then they will use that vulnerability to keep you on a string. Trust me, if it is as you say this is likely the whole reason behind giving you a greeting card or message on your Birthday.