How is this any different from the rules at any company, say Google or Facebook. Can you start a public political debate on the Facebook company chat? This just sounds like 37signals is all grown up and has decided to take the inherent risks in political discussion in the workplace seriously.
At least when I worked at Google a couple years ago, you could indeed start political discussions. Communications are very open at Google, and employee activism is quite visible. Political questions also come up in Q&A's, and some employees expect leadership to comment on societal issues/events. There has been some tightening in the past couple years, due to how loud it got in the last few years. But still quite open from my understanding.
One sample consequence was that open derision of Republicans was fairly common at Google, from both individual contributors and managers.
