I don't disagree with what you're describing at all. But some of the truly abusive self voting occurring involves a user making a post that simply says "test," followed by 100 comments with no more content than the numbers 1-100. Then they just go through and upvote everything as a means of "mining" Steem. I don't think there can be any label for this other than low value content, and I'm in favor of flagging it as it's against the spirit, intent, and long term health of the platform. This is very different from short replies, memes and emojis which I believe will be much more rewarded as short form versions of Steemit, such as Steepshot and Zapple fully launch.
On your second point that people are free to use their stake as they see fit, I also agree completely. But I extend that unconditionally to the act of flagging as well. If mining via self voting is valid, flagging those posts is equally permissible.
Good point about flagging. That's exactly why SteemIt is so awesome - we can all participate in shaping the community in ways we like, or don't like. For example, I flaggged some post that was promoting Islam the other day - because I think it is a bad idea and promotes things which are detrimental to individual freedoms. However, some rich Saudi prince could come by and "punish" me for having done that. I could cry foul all day long - but I would have brought it on myself.