They are injured birds that could not survive in the wild. I believe the Fish and Wildlife Service runs the program that places injured Bald Eagles in appropriate homes. I have a sad photo that shows a Bald Eagle trying to take off - but she has only one wing, and half of it is missing. Sadly, this tiger at the Dallas Zoo is caged. Hopefully, they will construct a more natural enclosure soon.
Cages often cause poor photos.
The trick is to get the lens close to the links while shooting the subject at the furthest possible distance.