##Rationale for Closing
Questions that say they are not about Ubuntu, by saying they are about how to do something on a non-Ubuntu OS, or in any other way, are closed as off-topic.
Lots of information that's off-topic for this site is potentially useful to Ubuntu users. Information about hardware repair is potentially useful to Ubuntu users. Information about general programming skills is potentially useful to Ubuntu users. Information about cooking is potentially useful to Ubuntu users. It's helpful to set a reasonably clear delineating line where we consider something off-topic, which we have done. When something is about how to use an unofficial Ubuntu derivative, it is considered off-topic for this site.
(The exception is when it is also about Ubuntu. For example, if you had a question about how to burn an Ubuntu installation CD in Linux Mint, I think that would probably be considered on-topic, just as questions about how to do that on Windows and Mac OS X are considered on-topic.)
You're saying this policy is applied inconsistently. That may be so (and if it is, that should perhaps be fixed, though doing so wouldn't necessarily mean closing fewer questions). But as far as I can tell, you have not identified any way in which the policy is applied inconsistently. Questions aren't closed because they're potentially useful to users of non-Ubuntu OSes. They're closed when they're not about Ubuntu. The question you've brought up as an example is potentially useful to users of Ubuntu and of other OSes, but it is not about Ubuntu. Other questions that are equally useful to users of other OSes, but which are about Ubuntu, are not closed.
Like other users of the Internet, Ubuntu users can search the web for information we need. We are not limited to this site. Users of other OSes (whether or not they also use Ubuntu) shouldn't feel limited to this site either. If this and similar questions are posted on the Linux Mint forums, Unix.SE, or elsewhere, Ubuntu users should still be able to find and benefit from it.
##Providing Constructive Assistance
In substantial part, this is in reply to the more recently posted UPDATE section of the question.
I agree that we have a responsibility to treat people respectfully and in a friendly manner, including when they have off-topic questions.
The question you've given as an example has a comment in it, suggesting another site where the question would be on-topic (Unix.SE).
Are questions that are off-topic because they're about a different OS being closed without a comment? If so, then I do see that as a problem. But if comments are being posted (1) explaining clearly why it's off-topic, (2) referring to a site where it's on-topic, (3) linking to the FAQ (since it contains information about the scope of this site and where people can go for help with unofficial Ubuntu derivatives), or some combination of those, then I think we are already providing constructive assistance to people posting questions about those operating systems.