You know, having worked right in that very group only a few years ago, I have to chime in here. This really has nothing to do with "liberal mindset" at Microsoft. In fact, the majority of "hardcore" gun owners that took their training and guns seriously at Microsoft were in the Xbox organization. Yeah, there's liberals there, but please don't act like the Xbox organization has "We Love Obama" posters everywhere. I'd say it's quite the opposite in that group.
To the point: The issue is likely about needing to create an environment that appeals to the broadest audience possible, and that includes _parents_ that don't want their children to be exposed to "violent imagery." Sure, I understand all the counterarguments ("block avatars for kids", "to heck with the gun grabbers", etc.), but the Xbox is a product that is sold and used
around the world. Most countries/cultures do NOT share our viewpoint about firearms, and many have VERY complicated rules about image "ratings." Seriously, it's
very complicated and, transitively, expensive for Microsoft to implement if they wanted a multi-country/multi-government filtering environment for avatars.
In the end, this was likely a simple decision that favored the family orientation that Microsoft wants Xbox Live to retain. It really means nothing. It's a video game box, not a conduit for anti-gun (or pro-gun) culture. Of course, if it still fills you with a huge amount of righteous indignation, just cancel your Xbox Live subscription and sell your Xbox. I hear that Wii console is really cool

I don't fear guns; I fear voters and politicians that fear guns.