No private planes are generally allowed to land at Moffett Field, an airport run by NASA. If you know Silicon Valley, than you know how close the location of Moffett Field airport is to the Google head quarters. It is ideal for the apparently very jet-set Google founders.
Google pays $1.3 million per year to NASA for this perk. They apparently also carry NASA scientific gear on the plans, which just sounds like an excuse for the deal.
More details on NY Times.