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. 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.
I get my computer taken away when I get an F, do I bitch about it, nope. I understand the reason I got it taken away and fix it so I can get it back. That's all it is too it and many parents do this to make sure their kid does good in school. You're about the only one here who thinks it's wrong. Thing is, it's not right or wrong, it's the parents decision and you have no right to tell them how to raise their child. After all it's not your life and it shouldn't matter to you.