Disclaimer: I used to work at Google and have some incidental knowledge of Google's recruitment practices but am not an expert.

Many of Google's recruiters (especially the sourcers who are looking for candidates) are temporary contractors.

These contractors generally have a tenure measured in weeks and they are vying for one of very few full time positions. This high turnover, low institutional knowledge, and desire to distinguish themselves would be my best guess at explaining why the "Do Not Contact" note is not being respected.