They shouldn't, but they do all the time. If a person writes a Hub based on specific research for that Hub (as opposed to being a Hub that's based on objective information, even if assimilated knowledge) the Hub should present both sides. Otherwise, it's essentially a half (or less) researched piece of writing.
On the other hand, if a writer makes it clear that his Hub is a matter of his own thoughts on the matter, combined with assimilated knowledge and/or previous research not specifically done for that Hub; I don't see a problem with presenting one side to an argument, along with whatever works of others back up that one side. News writing, objective reporting, historical accounts, objective analysis, etc. should always present all sides. Not all writing has to be (or should be) objective. Writers have thoughts, biases, ideas they want to present. I think the main thing is that writers be careful to present what they write in a way that makes it clear to readers whether the piece of writing amounts to "reporting" or presenting facts/research or, instead, amounts to the writer's presenting his own thoughts and whatever he has to back up those thoughts.
One advantage to having that "intimate familiarity with the subject" that Google mentions in its list of what "webmasters"s should ask about their content is that intimate familiarity allows the writer to recognize what sources appear biased, flawed and/or otherwise inadequate; so while I think it's certainly possible to produce an objective, research-based, article without intimate familiarity with the subject; I think people need to be extremely careful when it comes to seeking out several sources that contribute to being able to present all sides of a subject.
But, having said that, I think if a Hubber wants to present his Hub as "Here's my personal impression of, and tribute to/condemnation of, Margaret Thatcher," there's nothing wrong with that. I just don't think the Hub should be presented as "an informative article". I don't think the writer of that kind of Hub necessarily should be required to have lived in the UK to voice his thoughts/impression or to raise questions he thinks readers ought to be asking. Readers can take it for what it is (as long as they know, exactly, what such a Hub actually is).