[UPDATE: For the UC e-mail two days later, which says the contrary, see here.]
The UCLA Faculty Association blog quotes the e-mail:
UCOP’s Office of General Counsel has determined that third party lodging and transportation services, commonly referred to as peer-to-peer or sharing businesses, should not be used because of concerns that these services are not fully regulated and do not protect users to the same extent as a commercially regulated business. As the market matures and these businesses evolve, the University may reconsider whether reimbursement of travel costs provided by peer-to-peer or sharing businesses will be allowed.
Therefore, until further notice, please do not use services such as Uber, Lyft, Air B&B or any other similar business while traveling on or engaging in UC business.