According to the notices sent out, Turbine feels justified for these actions: "Recently we conducted an investigation into an exploit that was being utilized in Dungeons and Dragons Online: Eberron Unlimited, involving our recent Mabar, The Endless Night Festival. As a result of that investigation, your account has been identified in utilizing this exploit. Please be advised that use of an exploit is strictly prohibited by our Code of Conduct."
One thread concerning the bannings was deleted, although another persists. Players express disbelief over what they see as punishment for rules they didn't know existed. One player, Madmonkey2, summed up the collective disbelief: "If the player has no idea what is going on, how can they be guilty of abusing an exploit? I personally did not gain one single benefit from that event, yet I am accused to doing something (unknown) repeatedly to benefit somehow?"
As you may recall, the Endless Night Festival was plagued with issues in the preview, although the event appeared to proceed more smoothly on the live servers.
[Update: The forum thread we mentioned has since been deleted]
[Update: It looks like those bannings happened by mistake. According to Turbine, less than 1% of the player base was affected, but those players can expect the error to be rectified and to receive compensation as soon as possible. Turbine's Tolero has posted a statement on the DDO forums.]