Ideally, we would just reform patent law to mitigate this but outside of that, I think it makes sense for companies to incentivize their workers to file patents "defensively".
I basically guarantee that that statement carries no weight at all.
They have a very valuable patent portfolio, and there is a very high likelihood that Google will at some point experience problems similar to Yahoo, Microsoft, Kodak, etc.
Once that happens, shareholders will be clamoring for that asset to be put to use - if management hasn't already done so to prop earnings up.
And that's why it's so pervasive - the only practical defense to patent lawsuits is building up your own portfolio of patents for defensive purposes. But, since it is highly likely that your company will face situations that require you to utilize that portfolio in an offensive way, your previously defensive patents will require other companies to acquire their own 'defensive' patents. Ad infinitum.
Of course, they're not doing that for everything (e.g. PageRank patents), but to the extent that they do this for the patents they intend to use defensively, they might avoid this problem.