You're partially correct in that they are invasive and destroying the ecosystem of the Everglades. However the outbreak can be traced back to a reptile center that was destroyed during hurricane Andrew, not necessarily people releasing their old pets.

From Wikipedia:

Although the exact origin of Burmese pythons in the Everglades is unknown, it is likely that many were once pets released by owners who found them too difficult to care for.[8] However, the majority of experts concur that the python population grew particularly after Hurricane Andrew. The category 5 storm destroyed a python breeding facility, which released numerous snakes into the adjacent swamps.