If you want to talk about the phrasing, I think you're missing a subtle distinction.
He disagrees IN principle, not ON principle.
As in, he disagrees in principle, but not in practice. As I understand it, "on principle" is the construction describing taking a stand on a decision because of your deeply held convictions. "In principle" is for a decision you've derived logically from a set of abstract premises.
The first comment in the thread uses "on principle". The next one uses "in principle", but I think it is used there to describe a belief.
I could very well misunderstand, I don't know (or the usage above could be loose).
I guess part of my problem is that I have trouble with a construction like "In principle, I believe in the sanctity of human life. In practice, sometimes you have to execute bad people."
(I think) That's better expressed as a preference that people not be killed. It's a silly extreme example, but I'm having trouble seeing how else it would be used, in a mechanistic sense.
It's more of "my principles tell me that this policy is wrong. However, I don't have the ability to make it right or avoid using it. So until it's set right by someone who DOES have that power, I don't have a choice."
So to use your analogy, I believe in the sanctity of human life. But my government doesn't. I don't kill anyone, but I live in a country that does. The fact that my government kills people won't stop me from driving on the government-built roads.
However you want to word it, I disagree with T-Mobile and AT&T's net neutrality policies. But I'm not in a position of power to make them change that, so what can I do?
He disagrees IN principle, not ON principle.
As in, he disagrees in principle, but not in practice. As I understand it, "on principle" is the construction describing taking a stand on a decision because of your deeply held convictions. "In principle" is for a decision you've derived logically from a set of abstract premises.