This restriction, as described in the accepted answer, is meant to solve two problems. One of these problems is already solved, and the other one is a misconception. There would be no problem if this restriction were removed (especially as it's already not present on the main site), and there are problems the way it currently is.
The problem of generic, non-descriptive tags, like facebook, is a problem that applies to Stack Overflow as a whole (not just the Facebook mini site), and Stack Overflow already has a mechanism to solve it- meta-tags. Questions are already required to use one descriptive, non-meta tag to be posted. If facebook is a meta-tag (which I believe is already the case), that problem is solved, for the Facebook mini-site and the site as a whole.
The other surmised problem, as posited above, is "having only Facebook tags on a question doesn't help much to categorize things within the Facebook portal". Having only Facebook tags on a question does help to categorize the question, whether you're within the Facebook portal or not. Just because you're restricting the scope of your view to only show tags relating to Facebook, does not mean that there's no information in a tag that describes in what way that question relates to Facebook. Indeed, which specific Facebook-related tags it has are often the only way you can classify the question.
It would make equally as much sense to disallow tags unrelated to Facebook in this context as it does to require them. I could make the argument that I don't want to read questions about bugs in Google Chrome, that tangentially include its behavior on Facebook - all I'm looking for is Facebook programming questions. Such a restriction absurdly restricts the tag vocabulary, just as the current inclusion requirement absurdly extends it.
This restriction is harmful, because the only way to get around it is by adding irrelevant tags, which reduces the usefulness of the other tags - see Timmy O'Mahony's answer. It also makes it harder to post your question.
How you're filtering the tags when you're browsing shouldn't change your capabilities when you're asking.