And the ASPCA, with their never-ending tear-jerking ads about donating to save dogs lives, when they are pretty big on killing unwanted dogs [Position Statement on Euthanasia | ASPCA],

In many areas of the country there are more pets than there are appropriate homes. The ASPCA believes that unwanted pets deserve a dignified, painless death

seems to confirm your comment. It’s cheaper to kill these animals rather than use the donations to care for them.