Right is a subjective term. I don't think they're right. IMO taking away someones computer access because they did poorly in school is a wrong thing to do.
it's not as much as all access to computer as it is restricting his access to time wasters such as video games and other distracting material.
The parents know better than both of us. OP could very well be addicted to computer games and skips his homework and studying for computer games, in which case it would be completely logical to restrict access.
They might think it's right, and since they probably do, they think it's the best thing for him. I disagree, though. I think having a balanced life is essential to happiness, and having the things you enjoy cut out of your life so you're forced to focus on what you don't enjoy isn't only wrong but unhealthy.
At the same time I think what they've done is given Trogtor some rope to have the balanced life and so far he's not actually doing that. It's completely possible to play video games and have good grades, and so far I'm assuming Trogtor has abused the rope that's been thrown to him.
If he takes this opportunity to spend time studying and doing homework instead of playing video games, his grades will improve greatly. Obviously fun is an important factor in someones life, and I'm sure he will be granted access if he actually takes this restriction into advantage.
He forgeted up, and now he has to be rehabilitated so he can be trusted to live the balanced life you believe in. He should see this as an opportunity to improve, not a deprivation of fun.