I find it hard to believe that it is a licensing issue
I'm certain that's not!
I'm not aware of any good reason for Ubuntu hiding the fact that its
based on GNU/Linux.
I'm certain that they aren't hidding. If they're they do a very poor job.
Ubuntu comes with proprietary software, but it doesn't come with any flash implementation that's not Free Software (someone refered to proprietary flash in this thread), and many of those drivers and firmwares are what makes people able to use the rest of the sistem and applications as Free Software only.
Let me give you my example: I need to use wifi network and a proprietary version of Java in order to use my work network and work VPN. In some locations of my clients I only have wifi (and the free driver needs de proprietary firmware), and I haven't been able to use the Juniper Networks VPN client with free implementations of Java (I've no choive about using that VPN client). I don't use any other proprietary software in the computer. I'm not happy about using this trash, but it's an forced compromise I need to not use a lot more proprietary software.
In the past even RMS, made some compromises like this and I only make them when the rules aren't set by me, and when I can't have any influence. But I allways let people aware of this problems and periodicaly try, to set me free, of this proprietary problem. When I buy a computer I allways test it to make shure I can use it with Free Software only, but there're times I don't have a word in hardware choice (it's imposed by the employer).
Although Ubuntu is not a Free Software only distribution I don't think we should not recomend it just because of that, what we should do is recommend people to avoid using proprietary software, and only use under extreme situations (and under protest). Ubuntu enables a lot of people and organizations to use a lot more Free Software. And they've allways prefered to provide Free Software (proprietary drivers are only used if the free driver for that device fails) and proprietary software is on specific repositories (restrited repository and thir party repositories), so users won't install if they don't want so.
I allways mention GNU/Linux when I want to refer to the GNU/Linux distribution and I think it's very important that users are aware of Free Software and the need (their and ours) to use it I can accept that not every body feels the need to say that something is GNU/Linux in order to make people aware of software related freedom. I admit the possibility of using other strategies. Some strategies don't mention software freedom, I'm certain that those will fail. Others inspired in the carrot and stick principle might fare better (comparing with not talk about software freedom).
I admit that not every user/programer/whatever of Free Software cares about software freedom and they're free todo so (even if I would prefer and ask them otherwise). In such cases, our victory will be that they've their freedom and that some provide it to others. Some day, and somehow they will learn better, and we will reach their users, because we won't fail.
I would prefer that they would explicit mention GNU/Linux, or even better Free Software everywhere, and I might tell them that it's better for everybody to do so. But I won't "go to war", or even be hostile with them because of it, because it's useless to our software freedom goal.
with my best cumpliments to all Diogo