Do verb tenses have to remain absolutely consistent through a piece of writing?

Generally, I've been told that I should maintain consistency of tenses in my writing. That is, if I begin a piece of writing in the past tense, I should ensure that all verbs agree with that through the document. But as I read more articles, books, and other such things, I get confused by the verb tenses used by these other authors.

For example, I copied a portion of an article below. I believe it started in the past tense, yet the second paragraph switched to the present tense (it sounds like the...) In the third paragraph, the past and present usage is mixed (the word gave is in past tense, yet remain is in the present tense). I understand that the writing in the article is most likely correct. But why can it switch verb tenses and be ok while I was told in school by my professor to keep my essays in a single tense? Is there a guideline I can adhere to?

REPRESSED for decades, the anger burst like a summer storm. Rioting youths flooded city streets. The shaken regime granted hasty concessions: freer speech; an end to one-party rule; real elections. But when Islamists surged towards victory in the first free elections the army stepped in, provoking a bloody struggle that lasted until the people, exhausted, acquiesced to a government similar in outlook, repression and even personnel to that which they had revolted against in the first place.

It sounds like the recent history of several Arab countries: Bahrain, Egypt, Libya, Syria, Tunisia and Yemen, the states of the 2011 Arab spring, have seen some or all of the story unfold. But this is also, and originally, Algeria, a quarter of a century earlier—the first major political crisis in the age of modern Islamism.

A flurry of freedom in the late 1980s gave way to a vicious civil war in the 1990s that left as many as 200,000 dead and Algeria’s Islamists more or less defeated, but not eradicated. Today the country’s citizens remain powerless spectators to a continued stand-off between what they call le pouvoir—the entrenched oligarchy that controls the state, the oil money and the army—and the now-marginalised Islamist radicals, who serve more as a justification for ongoing repression than as any sort of inspiration to ordinary people.