If there is a policy, it will be abused.

Someone on Wikipedia decided to block a whole /16 from which my IPv4 address is issued under the pretence of an open proxy.

Open proxies are prohibited on Wikipedia. It's allowed to block up to /16, but time must be limited, and it's generally discouraged to block so much space in the first place. So, someone just decided to block the whole space, because they could, even though the space has little to do with open proxies.