I also don't believe anyone got only 1 B in their ENTIRE high school years.
At least two members of my graduating class fall under that category, as well as one of my professors here at school, I believe. It happens.
I think that rattler's method, if stated bluntly, is fairly reasonable. I'm sure that if grades became a chronic problem you would address the causes before resorting to months or years of arbitrary grounding. There's a difference between punishing poor study habits and neglecting your child's education. A lot of misunderstanding happens because most kids don't appreciate that parents often take a firm stance but still have best interests at heart; grounding is a steep penalty, but grades are a good metric to evaluate school performance, and in my experience with today's nothing-you-do-is-wrong educational attitude you only need drive and basic understanding to net B's. Sometimes parents get carried away with punishments - unless you come from a house with more than ten children, they usually don't have a lot of practice deciding fair penalties for raising a kid your age. But rarely do they do it out of spite or neglectfully. Unhappy kids are not fun to care for.
I'm considered to be smart, and I know a number of people that would say more of me, but I totally crashed and burned all through middle and high school. To a large extent, it was out of my control (dealing with a laundry list of health issues) but some of it had to do with poor study habits and a lack of motivation. My parents did punish me for certain bad grades, but they were also aware that my frequent illness was a part of the problem and always kept in touch with my teachers so they could tell the difference. I think that's the same case here.
As for your parents, Ant, I think they're full of bull and probably were just venting at you out of stress. I'm sure they're really happy about having such a bright kid. In all likelihood, they're too stressed about what they might be doing wrong as parents to notice that they didn't give you a pat on the back. Most parents I've met go really nuts over their kids. It's like the crazy cat lady syndrome, only silent and deeply ingrained in parental instincts.