What exactly is this?

I was surfing yesterday and found this article on BBC web-site. Through this article, I came to know about a tradition in United States during Thanksgiving dinner. They have a big celebration in the White House and turkey is the major attraction on dinner table. Roughly 300 million turkeys are raised annually, and around one-sixth of them are destined to Thanksgiving dinner plate. Two "lucky" turkeys among 2500 others specially raised for the White House dinner are selected by the President and are "pardoned". They are not cooked and are released. They are usually sent to a park called Frying Pan Park. I do not understand what exactly does it symbolise? In fact, I have some questions regarding this.

Birds are raised, not as someone who is supposed to enjoy life, but as the food. So the food scientists try to make sure that the product is as bulky as possible. Heart and lungs of birds raised this way do not support their body. They often die of heart attacks too! According to this link, "if a seven-pound human baby grew at the same rate that today's turkeys grow, when the baby reached 18 weeks of age it would weigh 1,500 pounds"! Now my point is, why not kill turkeys who have already been made so handicapped that they can not even bear their own weight, and why at the last moment change the mind and leave their life in the name of mercy, so as to let them die very soon, even a more painful death on their own!. In deed, most of the "pardoned" turkeys do not live long enough. They do not even remain to see next Thanksgiving day! Even for humans there is a provision of Euthanasia!

Another think which I can not understand is the terminology - "pardoning" birds. Have they already committed some crime from which, you being nice, are exempting them?