Oh, I meant to say that there are terms of service for Steemit, not Steem. Since Steem and Steemit are both created by the same group, I combine them in my mind.
The terms of service for Steemit specifically includes a term in section 17 that is relevant.
Cases of blatant plagiarism with no added value from the author and deliberate malicious misuse of the software that amounts to criminal intent are valid reasons to downvote, i think. Deliberate propaganda and misinformation is also a valid reason. Some might say that some of these are subjective, but ultimately I feel that the absolute truth can be known in all cases - though it might require a lot of research to find it.
Not at all, Steemit Inc. Is subject to local laws and as such could be compelled to comply with them or face termination. As such they could choose to remove accounts from Steemit and, although I am not familiar enough with the licensing for Steem, could potentially also be forced to effectively enforce laws on Steem too. It's a new technology, so the case law is not clear - i think - however, a blockchain is not an ungovernable system.
They have right to determine who does and does not use their services.
I am not aware of a single example of that logic being held up and supported in court, but I am not the world's leading 'legal' expert.