Hi – great question!
It’s important to note that these are not pages or posts in the traditional WordPress sense (so it’s not added thousands of database entries, for instance) – rather our events page accepts queries for specific dates and returns a list of events if there are any, or else a 404 status and a note explaining that there are no events.
We do set a tag to tell search engines not to index said pages and that seems to be in place when I look at the URL you provided – really then the information Webmaster Tools is providing can probably be considered as potentially useful for diagnostics rather than an indicator of a problem, as such.
That said, we appreciate this can cause an amount of concern and inconvenience and we’re actively looking at ways to further mitigate situations like these.