Domain registrars are providing a utility service, they are essentially "common carriers". Imagine if your phone company revoked your phone number, or the postal service refused to send you any mail because of the content of your communications? What if the DMV revoked your license plate because of a bumper sticker on your car? Domains are (at present) a key linchpin of freedom on the internet. If ICANN allows registrars to terminate contracts for political reasons, that's a huge blow to freedom online.