There's obviously some sense to that. Still, I think it depends somewhat on the group - and this just won't work for some. In both I play in, the "problem" players play purely for fun; their mistakes are definitely honest (if occasionally frustrating). Telling em off won't work (tried that): what happens is simply that they still make mistakes, but that these mistakes sometimes cost the party dearly. The players in question may be annoyed by their mistake, but they don't really improve.
I've played with hundreds of players over the years and I have never seen a single player who didn't improve. Not once.
It's possible that it could happen in a game where combat is rare, but in our games, there is typically at least one combat encounter per session, often 2 or 3.
This gives every player the chance to repetitively see their mistakes. They might not become tactical geniuses, but they have in every case gotten better and made fewer glaring mistakes over an extended period of time.
Now, there might be players who don't care and just want to see how much trouble they can cause (these are sometimes the same players who want to play insane PCs so that they can do whatever they want whenever they want), but these players are typically doing it on purpose and I have no problem hitting them with the DM bat.
On the topic of the rules, you can't make a melee attack with a melee weapon that you aren't wielding. The "correct" ruling isn't that the attack provokes, it's that the entire situation doesn't make sense and can't occur. You can rule that he used his ranged weapon - even though that obviously wasn't the intent, perhaps in some situations that's a fair punishment - and then he'd provoke, but that's definitely not somehow "correct" in the sense of necessarily following from the rules.
Not necessarily.
The player stated that he attacked, he did not state that he attacked with a melee weapon. If he had done so, then the DM should have said "You don't have a melee weapon out" and it would have definitely been an illegal action.
From the original post, the player did not even state which power he was using and even if he were using Sly Flourish a second time, that can be used for either a ranged or melee attack.
I don't think we can conclude that it was an illegal action, just a player who is not paying attention to the details.