Stopping managers exploit fresh graduates with intimidation and tough tasks?

Stop working 12 hour days, don't work on the weekends, and see what happens.

Well, people will exploit only to the point where you'll let them. You have the power to say no, to talk back (it's not a bad thing), to express your opinion. I've been in situation like this quite a number of times and I just tell myself: I'm a very talented developer and I can always get another job or work for myself if it comes to it, so I will not accept any exploitation. To do this you need confidence. As a developer, my way of getting confidence is being so good they can't ignore you (check out that book). Do your very best, learn every moment you get a chance. BTW, sometimes you can just talk directly to your manager and let him know how you're feeling. Tell him you feel like you're being singled out and you think it's unfair. It's the truth. If he can't handle that, and keep pushing you, just leave at the time you feel you should leave and tell him you have errands on the weekends. But use the weekends to your advantage by learning new skills. He/She can't physically force you to do anything.

There are management teams that expect the same even from seasoned teams. Sometimes you get lost in the challenges of the product, my recommendation is don't. There really is no need for continuous insane hours. Spikes, yes, regularly? No. It is a sign of poor management.

Sometimes founders will insulate themselves by hiring some engineering management to do their bidding.