Are People Responsible for Animal Shelter Overcrowding?

There are several kill free animal shelters in my area of the world, yet most of them are continuously booked up. The city pound is always full, even overcrowded. When it comes down to it who is really responsible for this?
Animals are often abandoned, without being spayed or neutered. These abandoned animals meet up with other abandoned animals and create thousands upon thousands of unwanted offspring each year. The animals starve, get hit by cars, and are abused by unsavory children and even adults. The ones that do survive often end up at the overcrowded city animal shelter where they will eventually be destroyed. It's all very sad and mostly preventable.
As Rachel Sa of the Toronto Sun points out in her article People are The Reason There are So Many Animals in Need, animals get in these positions thanks to the humans who aren't caring for them.
It is the responsible thing to only take on as many animals as you can adequately care for, it is also the responsible thing to spay and neuter your pets to prevent mass overpopulation. Unfortunately many people only think about these things after they hear sad stories like this one, and then let the thought go as quickly as it came about.