It could be just my opinion, but it still doesn’t count as permission if the announcement is made after the card is submission; nor does it count as permission to make one announcement and expect everyone at the event heard it. In order to add an email address to your database in an ethical way, you need to get permission from each person. If you’re using business cards to increase your database, I would recommend you instead print ballots with a special check box requesting permission to add each entrant to an email list.

For a draw, I’m only giving my business card for a chance to win.

If you were to do business card draws then make sure people know by both telling them if you announce it at your event and better still, putting that clearly where you are doing the promotion in writing.