If you've been reading Hacker News for the last few months you'l' probably have read two major complaints about the company.
1) their search results are getting spammier
2) if you're an employee on the inside, there's more politicking than engineering going on.
The +1 bonus addresses both.In terms of spammy results, if millions of people are applying upvotes to good results every day, true human-eyeball search QA will be working at massive scale.
And YES the spammers botnets have probably already started +1'ing their spammy links, but hey, answering that is part of the bonus. (I can think of a few algorithms already that might actually further identify spam by finding 'unnaturally correlated voting blocks)
+1 directly improves Google's core business, which is finding good results in web search. Having 25% of your bonus tied to the success of Google's core business is appropriate.
The second major issue I've read about (nearing the point of deluge) is the problem with 'internal politicking'. It happens at all mega-corporations I'm sure, but how in the hell do you address it? Individual departments work against certain other departments, they willfully work a little slower or protest a little too much and some projects that didn't deserve to die do.
The complaints about Google's ability to innovate are evidence of this stuff. The internal middle managers' squabbles.
Well what better way to ensure that you cut through the corporate squabbles than by looking at all the projects you've got going on, picking the one that you think has the best chance of helping the bottom line and saying to everyone in the company "you can either help or get the fuck out of the way. 25% of your bonus is tied to this project"
If morale is going to be take a hit if this project fails, then the answer is don't fail.