How about because of equal protection before the law? The government should be negotiating on behalf of all the people it represents.
"The most common type of tax-exempt nonprofit organization falls under category 501(c)(3), whereby a nonprofit organization is exempt from federal income tax if its activities have the following purposes: charitable, religious, educational, scientific, literary, testing for public safety, fostering amateur sports competition, or preventing cruelty to children or animals. " from http://en.wikipedia.org/wiki/5...
When consulting I would often do work at discount rates for a medical clinic which had a sliding scale fee setup and for a law group which defended death penalty cases for indigent clients.
Non-profits do a lot of good stuff, people should help them. Stop being so miserly.
"Gravitation cannot be held responsible for people falling in love." -- Albert Einstein