You're certainly not the first person to ask about
Birthday Present. You would be right, but it's written in a very specific way to allow something counter-intuitive (pull a card without giving the Shadow player any choice) to happen. Two relevant bits from the rule book to start with:
costs for an action are usually listed before the word "to" (so the action takes the form of "pay X to do Y," with X being the cost and Y the effect).
...
If the effect of a card or special ability requires you to perform an action and you cannot, you must perform as much as you can and ignore the rest. (See limit.)
As you know, if you cannot pay the costs in some way, the effect is ignored. The cost of
Birthday Present is simply "add a burden." Once that cost is paid, we move on to the effect. If there are not 2 different
events in your discard pile, the "perform as much as you can" stipulation has us pull as many as we can (either none or one). Your opponent then chooses one card (if able), and that card is put in your hand.
There are a few ways it could've been written to prevent this, but the card's designer wanted to allow the Free Peoples to always get a certain card back if there were no other
events in discard. For example, it could have said "Add a burden
and choose 2
events with different card titles from your discard pile." or "Choose an opponent and make him or her choose 1 of those cards for you to return to your discard pile. Take the other one into hand." The wording is very careful to allow manipulating a single card and remain consistent with the rules.
As you may imagine, there was confusion when the card was released too. Decipher released a clarification for the card explaining the intent in the Current Rulings Document from just after it was released up until Comprehensive Rulebook 4.0. This is taken from the August 9th, 2005 CRD available at
http://lotrtcgwiki.com/wiki/_media/crd_2005_0809.pdfBIRTHDAY PRESENT 10 R 104
If the Free Peoples player has only 1 event (or up
to 4 copies of the same event, but no other
events) in his or her discard pile, that event is
taken into hand.
Hopefully that clears things up. I know it isn't a satisfactory answer, but the card remains consistent with the rules even without Decipher's clarification.