Interesting, I thought about what you said for a little while, then looked it up because I couldn't think of the best way to explain it. I think this is the clearest explanation I found: principles (noun plural)---one's own personal rules or standards of behaviour. It is against my principles to borrow money. in principle --- in general, as opposed to in detail. on principle- because of one's principles. I never borrow money, on principle. (source: http://www.thefreedictionary.com/in+principle) Think of it like this, if you rephrased it: In principle (In general), Ruth approved of the idea. On principle (according to her beliefs), Ruth approved of the idea. She felt more deeply about it in the second phrasing, would be my interpretation, but I think it is a minute difference in meaning.

Mr Irving was correct. I'm sure he'll be glad to know that Maria and I agree. In principle: as a general idea or plan, although the details are not yet established or clear: "the government agreed in principle to a peace plan that included a ceasefire" {couldn't find the source link} On principle: phrase If you refuse to do something on principle, you refuse to do it because of a particular belief that you have. He would vote against it on principle. His father, on principle, did not like to hear requests for money. {Harper Collins}

If he had meant "on principle", her style is clumsy and tautologous, and he could have simply said: "She approved of the idea/principle." Otherwise, he meant "in principle". I share your initial reaction to the text.

Yep, I would go for "in principle".