It is currently intentional, though it might be worth reconsidering. I posted an earlier question to consider the tagging convention for Ubuntu versions; that post includes a fairly current breakdown of the major OS tags that are currently in use on SU.
On Super User, I think tagging questions according to content provides the most useful categorization. I think it's more useful to devote as few tags to the OS as possible, unless the question is particularly about operating systems. This leaves the other 4 tags for whatever the question is really about. Customizing a theme? Image editing? Media players? Media format conversion? Partitioning a hard drive?
I think most questions about solving a problem with an OS will be specific to the version of the OS. Questions asking for software recommendations may be more general, but if you've got a problem with your network driver, it's pretty likely your question and its solutions will depend on what version of your OS you're using.
We certainly could stuff all Windows 7, Windows Vista, Windows Server 2XYZ, and Windows XP tags into [windows] and [windows-server] tags, and likewise for Ubuntu, OSX, etc. ([osx] is already the defacto OS tag for Mac OS X; the [osx-codename] variants are relatively scarce.) It would be a simple process to batch-retag the various Windows and Ubuntu tags into their more general category; doing the opposite (splitting the OSX tag into the proper version-specific tags, or making sure everything tagged [windows-7] is also tagged [windows]) would be completely manual.
Keep in mind the tag-search wildcards (eg [windows~] should allow you to search all the OS variants at once. This doesn't seem to work quite right when you're trying to filter by a cross-reference of tags (searching for [windows~]+[usb] should find questions tagged [any-windows-tag] and [usb], but doesn't)...