Agreeing with McGraw, Firestorm, DM, Pillo etc
The punishment, IMO, is nowhere near harsh. Are you used to having everything your own way? Most parents would've taken it far worse (the first time).
It seems like you're trying to regain trust just to 'go and do it again', which is completely retarded. While weed isn't as bad as most people make out (some of my friends do it every now and then, and I'll admit I have) it's only going to lead you onto harder drugs again eventually if you do it a lot. You know you'll be talked into it by other people if you don't just stray onto it yourself. Judging by how seriously your parents have started to take this, it'll just be a lot worse the next time you get busted, and let's face it, you will.
It's best to just deal with the punishment and when your parents think the time is right, they'll let you off the hook (but still be more watchful over you). Don't go and do drugs again, you'll only ruin your life next time and probably end up on the streets. Unless that's what you want, then go for it.