You make the false assumption that people in different divisions of a very large company would be more likely to help each other outside of the corporate reward structure than they would be to help someone outside the company, who they might just happen to have personal or social obligations to.

If there were an unscrupulous person in the gmail group, he would be more likely to break company policy (and, I think, the law) with someone who does not work for Google than with someone who does, unless it were motivated by someone up the chain at Google itself.

Saying that someone at Gmail is sharing your secrets may be a worry. That they would share them with Google Ventures is far less likely than that they would share them with someone else entirely. And frankly, if someone were to try to profit from stolen information, they'd be looking at hedge fund manager emails and investment banker emails. The very slight edge you might get from seeing some VC's email is worth less than a cup of coffee at Starbucks, if you risk-adjust it and discount it back to the present.