It's kind of justifiable in the sense that it's more convenient in the usual case. You say `asciinema rec`, do your thing, and then you have a secure URL to share with somebody, and you can make it public with a couple more clicks.

Actually I think this has changed, because when I tried this just now, it asked me

   (s)ave locally, (u)pload to asciinema.org, (d)iscard
   [s,u,d]? u

which is a little more friction but probably a safer default.