Historically, naming rights for an OS go to whoever actually puts together and distributes the complete system. For instance, if a workstation company licensed Unix from AT&T and ported it to their workstation, they got to name that OS whatever they wanted. A couple examples of this were Uniplus+, which was UniSoft's Unix, and 386/ix, which was Interactive System Corporation's Unix. Both were Unix systems--they used a Unix kernel and Unix utilities--but that wasn't their names. Half the fun working at a Unix workstation company in the early '80s was thinking of a neat name for your Unix port. :-)
For the complete systems distributed by Canonical, Red Hat, and the like, they are the ones who get to name the operating systems that they distribute. Ubuntu calls their OS the "Ubuntu operating system". Red Hat calls their OS "Red Hat Enterprise Linux".
Yes, they are also GNU systems, but if we want to be historically accurate, the most correct way to view this would be to view "GNU system" and "GNU/Linux" as specifications for a specific Unix-like userspace and for an OS that runs the GNU system on a Linux kernel, respectively. The Ubuntu operating system complies with the GNU system specification and is a GNU/Linux system, but it is named Ubuntu operating system.
It's called Linux because we call it Linux. That is how language works. It doesn't matter about the official name or the etymology, if the population typically uses a certain term for something, that is the "correct" term.
Now there are lots of arguments for preferring an atypical term in some situations. Official names in more formal contexts, genderless nouns to avoid excluding people, and indeed "GNU/Linux" to promote the GNU project. But these are choices we make, and are not something to be forced on people who don't necessarily agree with the reasoning behind them.
The GNU project had built all pieces of the OS, before convincing Linus Torvalds to release his proprietary kernel code in the GPL license.
"Since the initial release of its source code in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to its state in 2009 of over 370 megabytes of source under the GNU General Public License."
And by the way, the GPL was first written in 1989, so you could hardly argue that in 1991 many people understood how free software should work or which are the longterm consequences of using a certain license for your hobby project. Cut Linus some slack, he released source code and eventually used the GPL, right?
It's great we're talking about the kernel codebase then!
So is X.org, the KDE project, the Mozilla project, and a number of other projects which aren't as well-known. So should it be X.org/KDE/Mozilla/OpenSSH/GNU/Linux? If not, why not?
The Android system uses the Linux kernel, but should people refer to the OS as "Linux" or "Android"? I think even calling that as Linux is fine, as long as people understand its just the kernel that they are calling out. The OS is either GNU/Linux (desktop/server OS) or Android (the mobile phone OS).
The convention has generally become that Linux-kernel operating systems are called "Linux". But you have options if this offends you so. If you don't like that, Debian's over there and willing to play along, and if you really don't like it, I hear that GNU has a kernel of their own rattling somewhere around gnu.org.
Years ago, I used to joke with friends starting with ubuntu, that they were using really adobe/nvidia/broadcom/micrososoft/oracle/etc/gnu/linux.
you GNU terrorists are funny, thinking you can go and claim others ppl work by attaching "GNU" prefix to a projects name and lying to everyone how it _is_ gnu. Unfortunately Linux is only GPL2 (and that too was a mistake as we can see from the above example - Linus said something about it too as I recall).
Don't be ridiculous.
[edit]: spelling