on principle
1. On moral or ethical grounds. As James Russell Lowell wrote about Alexander Pope in 1871, "There was a time when I could not read Pope, but disliked him on principle." [First half of 1800s]
2. According to a fixed rule or practice. For example, The police were locking up the demonstrators on principle. [First half of 1800s]
3. on general principle. For no special reason, in general, as in Dean won't touch broccoli on general principle. [First half of 1800s]