You're still missing the point: without a ban, there rarely is any internal debate because only the prevailing company opinion gets a voice. Changing company policy doesn't alter this dynamic. If a company shifts from being liberal to conservative all that means is that conservatives get to use the company as their soapbox instead.

It's better to just not let anyone use the company as a soapbox at all. If they have disagreements about the culture of the company, there are plenty of reporters looking for a scoop.