All of those things are "dangerous" in this sense.

It is arbitrary, but that's how Ruby works; there's a gray area shared understanding of concepts. The "principle of least surprise" is literally about Matz, and Matz alone. Yes, something may be surprising to you, or not, but it's not about you. This shared taste/understandings/whatever is just how convention works. (Or at least, it did when I was involved with Ruby; it's been a couple of years so I'm out of touch!)