@r works only for players if it doesn't have type=

The command
/kill @r
only kills a player (in singleplayer me), it can't target a random entity.
The command
/kill @r[type=ArmorStand]
actually kills a random armor stand. But if I want to kill a random entity in the world (or a random one that fits a certain condition), it isn't possible (except, of course, I summon an armor stand with a name at every entity and execute at a random armor stand and and and...).
I think this is a bug, since with @e it always works.