It was wholly a pleasure to get your pointed question about a phrase I used the other day about those who "tend to confuse ideology with principle."
What's the difference, you asked, and would I know?
You have a point, for one man's ideology can be another's principle. Someone whose politics we don't much like we call an ideologue, while someone we agree with is of course a person of principle. But your point has its limits, for there's a reason one word is usually offered as criticism and the other as praise.
Ideology is a modern term (for ours is an age of ideology), and we even know just where, when and by whom it was first used:
In France in 1796 by one Destutt du Tracy. The concept of ideology is one more unfortunate legacy of the French Revolution. Even though definitions of it may vary, the word has come to mean a set of ideas, usually in politics, that narrows the mind while enflaming the passions. Which is why it has become a term of opprobrium rather than description.
Think of the mobs during the Great Cultural Revolution in China. Armed with their little red books of quotations from Chairman Mao, they set out to destroy an ancient culture. For an example of ideology in even bloodier action, think back to the Khmer Rouge and their killing fields in Cambodia.
Naturally enough, the Frenchman who would coin the word ideology had been imprisoned during the Reign of Terror, a victim of ideology himself. The history of words, like history itself, is just full of delicious if not always pleasant ironies.
By now, only professors may use the term ideology in its original meaning -- as a guiding group of ideas or worldview. In general usage, it's come to mean something more rigid and intolerant.
It is easier to illustrate the difference between ideology and principle than to define it. Just compare the writings of Karl Marx, who spoke proudly of his ideology, to the Federalist Papers, which are blessedly free of it.
By their fruits ye shall know them: Compare the French Revolution, which became the historical template for modern revolutions, with a quite different one -- the American Revolution. One would culminate in a Reign of Terror followed by Napoleon's dictatorship, the other in a republic and the Constitution of the United States.