I'm guessing that you haven't talked about this with anyone who is into BDSM. Just because you witness something that you think is violent abuse doesn't mean that you're right.
Conversely I've seen perfectly pleasant looking couple who engage in constant verbal abuse disguised as pleasant commentary. Most people won't recognize it for what it is. Go read http://www.amazon.com/The-Gentle-Art-Verbal-Self-Defense/dp/... and you may do better than most people.
Be very, very careful about judging what happens in other people's relationships. It may be bad. Or you may not know. For real.
Get dozens of book recommendations delivered straight to your inbox every Thursday.