I guess it's generally just that it's regarded as fairly idiomatic Ruby. bbatsov's popular style guide, for example (https://github.com/bbatsov/ruby-style-guide#if-as-a-modifier):

The same applies to while/until loops, which can lead to quite succinct code.

I've been using it for a long time, so I don't find it confusing at all – but I agree it's pretty rare to find, so I'm not surprised to find out some people would rather not see that!