Speaking as someone who has made such blocks but only later found out the off-wiki context, sometimes they're absolutely necessary to prevent disruption. The people facilitating the event these events (I've done that too, back in the days when Wikimedia UK actually did stuff) need to be careful, especially if the event involves multiple people editing a small group of articles from one IP address because the people who respond to those edits on-wiki don't know there's an editathon happening if nobody tells them.
We also see a lot of abuse coming from places like public WiFi hotspots. University WiFi often gets blocked because of trivial problem edits but public WiFi hotspots are sometimes used for more sophisticated abuse - for example, we have problem editors who drive between coffee shops/fast-food outlets/other places that offer free WiFi solely to evade blocks and make disruptive (sometimes extremely problematic) edits to Wikipedia.
Harry