If you run a messenger service, you aren't entitled to decide that select groups can't use your service. You can't decide that you will monitor the messages, and only deliver those messages that you approve of. You don't get to decide that you will deliver partisan messages that favor your position, and just lose messages that support the other side.
As an email provider/carrier/whatever, Google has a responsibility to pass the messages on, unless and until they actually violate some law.
How about if your phone company listens in to your conversations, and cuts you off when they disapprove of your conversation?
Now - you can twist a pair of panties into any kind of a wad you like, but you cannot twist morality and ethics enough to justify censorship of private communications. Nor can you justify political communications. Can't even justify censorship of business communications, until those communications violate a valid law.
Morally and ethically, you have a point - but legally, no. Telephone companies in the US have specific laws regulating what they can and can't do - but if Google decided that from now on, any email containing the word "viagra" would get blocked from Gmail, that's up to them. Probably not a useful choice (spammers already use workarounds like "\/iagra" anyway, and the occasional legitimate email would get caught) but it is theirs to make. Indeed, this very site has a few rules to reduce spam and misuse - so you can't post very long words without getting random whitespace added (to combat the old "page widening troll"), you can't post more than a certain number of messages in one period of time - all rules they are perfectly entitled to adopt and enforce, since it's their own site/service.
Someone posted here earlier that the domain looks quite "spammy" on some of the heuristics Facebook and co probably use internally: it wouldn't exactly be the first time legitimate content got caught by a spam filter. More likely than a conspiracy theory about Twitter and Facebook being so determined to stifle criticism of TPP. As of right now, stopfasttrack.com is not listed in Spamhaus's database; probably someone got over-enthusiastic promoting it, and some of those messages got reported as spam. Nothing new there, either.