Private cars / limousines / independent sedans are not regulated the same way as taxi cabs under the specific assumption that they are scheduled in advance (which changes both the information and market dynamics, often involves specific drivers, and provides more opportunities for strong contracts to be in play); this is actually required by law in some jurisdictions, including DC.

> “It happens to be D.C. law," Linton reiterated about the situation with Uber. "D.C. law prescribes that limousines must enter into an advance contract with the passenger.”

http://www.tbd.com/blogs/tbd-on-foot/2012/01/the-kojo-show-b...