I disagree, most **windows** users dont differentiate, this is mostly because its compiled as one monolithic beast with little ability to swap out parts.
Us linux users, however, recognize for us the line in the sand isnt so clear. When we do not wish to differentiate between userland and kernel we use the name of the distrobution. When we do wish to make the distinction we say "Linux" if the userland isnt important to the context.
Making the distinction of OS is certainly a fine thing. But on linux that distinction is far more fuzzy than on windows due to its modular nature and the fact that there is no hard line between what is built in or not.
Thats exactly why if i have any wish to refer to my OS as a whole then I reference my distro, its the only sure way to make sure the entire grey area is included.
The very idea of there being an "OS" seems outdated to me on linux. All you have is a kernel and a huge array of userland that sits on top, collectively making the distro, that is all.