A lot of development inside Google is done on Macs - you see this at every developer conference.
It's possible that Google have had to deal with a virus outbreak on Mac already and now are taking a proactive approach.
Google has upgraded its popular VirusTotal analysis tool by adding an Mac OS malware uploader in a bid to better understand increasing attacks against Cupertino's fruity 'puters. The tool has been made available for OS X 10.8 and 10.9. Malware and suspicious URL samples uploaded to VirusTotal are checked against 52 anti- …
If that's true, and they think OSX is an answer to this, they're bigger idiots. I suspect the "security" is a retroactive justification for not choosing Windows, when the real reason is a preexisting corporate cultural bias against Microsoft products. It's not like Google wouldn't have good IT staff...
I'm predominantly a Mac user, but I also use Windows 7 and 8, and these are just as (in)secure as OSX, and the anti-malware security measures on both OSes are just as easy for a busy user to click through.
The only difference between the platforms I can see is that the attack vectors in Windows are exploited relentlessly, whereas those in MacOS X have been largely ignored to date. ("attack vectors" doesn't just mean security holes, but things like website popups styled to look like system dialogs, or email mesages styled to look like Outlook's UI; these stick out like a sore thumb on MacOSX, but are less noticable on a Windows system).