Because someone once asserted it on here, and some people liked it enough to repeat it. It's got no basis in fact; the armrests are a free-for-all.

As for the soragami, if it protrudes out into the seat well itself on the other side, I doubt it will be welcome. That said, the basic idea is kind of sound -- a few times when I've hit "leaners" at shoulder level, or folks who are over-assertive in taking the entire armrest, I've found a copy of the inevitable in-flight magazine handy to shove between them and me.