Why the Concept of Billable Time is Bad for Creativity

I have worked at companies that had billable goals and at companies that did not have billable goals. I find that I work much longer hours when I do not have a billable goal. It may be a psychological ploy but if you have to work by the hour on various projects and you are in a position in which you do not get paid overtime then any hours that you are not billable seem wasted hours. Conversely, if I am paid a flat rate of 40 hours per week then every hour of the day seems as valuable as the next hour of the day and I find myself working again 12 to 16 hour days because I want to get the project done and move onto the next project.