Maybe the news reporters aren’t giving a complete picture, but I’ve never heard of this situation being a year-round issue. Generally, places that have these huge garter snake dens will have a swarm in the fall and another in the spring as the animals go into hibernation and come out of hibernation. During the summer, I can’t imagine that there would be enough food for more than a few snakes to live around one house. Furthermore, males tend to stake out territory, so they would not hang around together. In the winter, they aren’t going to be anywhere except in their hibernation dens. They aren’t going to come out of the rocks to climb around a house where they won’t find food.
The other odd thing is that someone who enjoys keeping snakes as pets would not want this situation. Wild snakes tend to carry snake mites, diseases, and parasites. No one wants his pets exposed to that stuff.