Because Google employees generally have latitude to do whatever they want with their Internet connection at work. Even bad OSM edits.
If we start blaming whoever an IP resolves to for vandalism, you're going to find that pretty much every major corporation has vandalized Wikipedia. (When I worked at BAC, our proxy was perma-banned for vandalism.) Not to mention the ISPs; Comcast is a notorious Wikipedia vandal (by this logic).
>Google honestly knew nothing about it...
If Google Management knew of this, wonder why they would let this happen from Google IP addresses...