Ah but...
If you develop an idea away from the office you could then sell it to Google or the highest bidder.
If you develop the idea at work it's already Google's property!
Googlers have clarified that the 20% time on personal projects is from you free time, not work time. Aka, you don't get an easy Friday, you get to do work on Saturday too (on your personal project). If you are looking for an easy job and many leaves, a company like Google, Microsoft or Apple is not for you (and certainly not for me, lol).