The infamous CP card is usually played by backwards-minded conservatives but it was, is and keeps being a potential big problem when you open your Wifi to the public. Also other criminal online activities.
So, having an open WIFI is like painting a huge target on your back, albeit a somewhat smaller one when media companies are removed from the list of potential trouble makers.
(See also: http://www.lawblog.de/index.php/archives/2016/05/12/dein-wla... (in German), for a defense lawyer's perspective)
It's not too dissimilar from a shop owner being forced to give up security camera footage -- the owner is not being targeted in the investigation, and there's plenty of unrelated footage, but there is a high likelihood of relevant evidence existing.