A lot of research has gone into what makes people behave this way. Read about "Tragedy of Commons"[1] and "Diffusion of Responsibility" [2] for more insights.

The net-net is that people rationalize their actions using whatever means necessary.

1. https://en.wikipedia.org/wiki/Tragedy_of_the_commons 2. https://en.wikipedia.org/wiki/Diffusion_of_responsibility