Unaltered pets in particular have a desire to roam. Pet theft is also a daily occurrence. Well behaved pets are sold for research. Animals such as purebreds, birds and exotics are valuable and are stolen for resale. Even horses are frequently stolen and sold for slaughter.
Most shelters destroy the majority of pets they impound.
They are destroyed only because they are not identified. Injecting the AVID microchip is just like any other injection or vaccination. Anesthesia is not required or recommended since there is minimal discomfort with the procedure. Not at all. The injection creates only a slight discomfort - most pets don't even react to it.