Hourly workers, and professionals paid on the basis of billable hours are less inclined to engage in unpaid activities than those who draw salary because they "develop a particular psychology of time, and come to make different decisions about time use" depending on how they are compensated. Salaried employees do not usually make more money by putting in longer hours, while those paid by the hour often make additional amount by working overtime.
Each person decides for himself whether to do volunteer jobs or not, but chasing the dollar all the time is rather a stressful way to live. Many hourly workers worry about losing all that money they could be "billing" either for recreational or volunteer activities, while there is plenty of evidence that people who volunteer are much happier. Some companies already recognized the importance of volunteering and give their employees paid time off to do charitable work.