Lyft and Uber try to get around this by describing passenger payments for these commercially offered rides as 'donations.' However drivers of these services are not being paid for their volunteer work, they are working for these services. They are working for the money, and in this way it's not a donation.
Finally ridesharing is a specific gov't defined catagory. Only when the costs are ridesharing costs ($0.565/mile or less) can they be viewed as non-commercial.
Ridesharing is defined by the US Government (MAP-21 bill) as offering the use of seats in privately owned vehicles to other passengers on a cost-reimbursement basis only. H.R. 4348 (112th): MAP-21
So any fee the exceeds costs-reimbursement is income, and these drivers are working, and they are doing so commercially. So they are commercial drivers, getting paid to drive people around. Ergo they are taxis, and they are required by law to be regulated like taxis.
And why legislate taxis? Because of the commercial abuse of the common carrier, in this case the roads. All commercial use of common carriers - roads, canals, highways, etc - they are all regulated to prevent abuse.
What does this abuse look like? Too many users.
What does Uber and Lyft allow? More users of the common carrier.
This is not a new battle, truckers, couriers, ship companies, all fight this - what's new is the social distribution that new technology has empowered.
For more discussion see -- Hill Brothers Chemical Co. v. Superior Court of Los Angeles (2004) DJDAR 13414
http://www.bledsoelaw.com/articles/treppa_michigan_trucking....
In part, from Quora discussion.
http://www.quora.com/Ride-Sharing/How-are-Lyft-and-Sidecar-l...