I got something like that using Panera Bread's wifi a few months back when I tried to access keepandbeararms.com. The firewall sent me a message that siad something like "Blocked. Reason: weapons" The other day I tried again and it was fine now.
My guess is that these places don't decide which particular sites to block, but probably work off of a list provided by someone else, possibly the software vendor.
If you think it worth your while, you could write a letter to the appropriate company office explaining that they are blocking legitimate political discussion and that they should remove the offending sites from the list.