As for the banning thing, I think Arenanet was pretty leniant about this. They didn't perm ban anyone who bought one of those exploited Norn cultural weapons a few small times (those got 72-hour banned). We're talking about people who bought and sold them THOUSANDS of times, thus messing up the economy (most likely buying store gems with all that gold). These guys went in knowing full-well that they were exploiting something broken.
The reason why Arenanet didn't simply do a roll-back is that they most likely wanted to put out a message to players that they were taking a hard stance on people abusing exploits. In the end, we players who simply want to play for fun benefit by having less assholes messing up the game economy in various ways (possibly inflating the gold --> gems so that the cost of gems via gold is outrageous). I know that after this episode, some guildies decided to err on the side of caution and report various exploits (sigil of speed perma-quickness for example) instead of using them for gain.