Why do we allow them?
Because we're Gamers, not Pedants.
#Because we're Gamers, not Pedants. Okay, well those of us posting on Meta probably are pedants, but we should endeavour to restrain our pedantry insofar as it risks driving away users.
The thing is, most of the time, when someone asks for a 'guide to x', they're really asking 'how do I get through x' or 'how do I use x well'. And if you look at most of the open questions with the word guide in them, you'll find that most of them have pretty good answers. Some of those answers link offsite, others are entirely self contained, but nearly none of them end up as 'lists of things' of the sort that we have trouble with. For instance, this answer is exceptional, and doesn't link offsite at all. Just because the asker used a word you're not happy with to describe his problem does not mean his question is invalid in any way, or even, I'd argue, in need of editing. Our users are pretty damned good at knowing how to properly respond to these questions for the most part.
Now, some requests for offsite resources really are recommendation questions of another flavor, and those are problem questions, and we're generally pretty good at closing those.
But, we really don't want to get into the business of closing and deleting questions over a failure to invoke the magic words (or not invoke the forbidden ones, as the case may be). In most cases, the intent of the asker is pretty clear, and if there are other issues with a question, as was the case with this mornings tempest in a teapot, then yes, by all means, clean it up to remove the word 'guide' if it annoys you so much. But to start a witch hunt over this seems exceedingly silly and shortsighted to me. The proper way to deal with these questions is to encourage and reward quality answers, while discouraging and downvoting low quality linkdumps and lists.