My wife works at Google. We both live in San Jose, and she frequently gets the shuttle to work in the Mountain View office. For tax reasons, I'm not allowed to get the shuttle with her to or from work, so I'm pretty sure that some if not all perks are offset somehow. Various things like the holiday gift (this year it was a galaxy nexus) are also offset for tax on your payslip. Net it works out at zero to you, as Google puts extra money into your paycheque to cover the extra tax charge.
Allowing non-employees on company transportation would threaten to make furnishing the transportation to employees a taxable event. See the same link that I posted on a sibling comment about food, just search for "transportation".
