You are exactly correct.
Microsoft isn't worried about Linux because Linux doesn't compete directly with Windows. Microsoft won over the US corporate world about a decade ago with a set of business and office tools that integrate the entire corporation together. Linux is getting there but they're about 2-4 years away from being where Microsoft was in 2000. You can't find many US corporations where you don't have a Windows computer on every desktop connected to Windows based email, directory and web servers.
This corporate culture that promotes everything Microsoft spreads to other non-Microsoft areas. Even though Linux and OSS has made progress in these areas of late, non-Microsoft companies must carry Microsoft computers for compatibility with the rest of the corporate world. Microsoft makes the standards and everyone else has to follow.
In addition, US corporations are like aircraft carriers. Once something is adopted, its unlikely it will change course anytime soon. Microsoft got there first and is now reaping the benefits. Most US corporations still use decades old technology because its too costly to switch. Any move to something like Linux would have to be phased in VERY slowly.
Linux may be approaching Microsoft in terms of functionality and ease of use but it has to convince large corporations that Linux is superior to Windows before there will be any effort to switch. As a result, Linux is still and probably always will be a niche market in the US.
In my experiences with corporations, projects that were Unix based have either switched to Linux or are in the process of switching. I've yet to see a Windows based project in my area switch to Linux. Since I support Linux based software, I've found that in many cases, being Linux only is a barrier to entry into some corporations because they've spent tons of money to develop a local support staff around Windows and are unwilling to retrain personnel to do Linux. On the other hand, a Linux based company must also support Windows so its less of a barrier for a Windows only company to sell its products to a Linux based company.
In Europe and especially emerging economies, where corporations and governments have yet to adopt the "everything Microsoft" mentality, Linux is making strong headway. I suspect in 10 years, we'll have a very similar split between the US and the rest of the world to what we see in many other technologies. Europe and Asia will be primarily Linux based and the US will still be Windows based.