I think others have covered the legal aspects here, and touched a little on the moral one. I think the heart of what you're asking is:
"Do I have a right to be pissed off?"
Yes. Don't buy in to the "rockstar developer" hype; If work were fun, people wouldn't get paid for it. You're not a family, you're not screwing anyone over. Being asked to do more than usual is an indicator that someone has over-promised somewhere down the line; and unless it was you, you're not duty bound to fix their mistakes (if we're giving the benefit of the doubt. I've worked jobs where it's baked in to the estimates.)
I recommend looking into "Adrenaline Junkies and Template Zombies: Understanding Patterns of Project Behavior". It's essentially a big book of bad company tropes. If you see yourself nodding along with too many of them, it might be time to abandon ship.
"What can I do about it?"
First and foremost: You could just not. Don't let your manager browbeat you into something you don't want to do. Unless they're literally about to make it a fire-able offence, don't bother. Even if you're in America, and working in an "at will" state (god help you), it costs quite a bit of money and time to go through the recruitment process for someone new, so the worst case scenario is that you get a "first offence" style talking too if they view it as mandatory (though please take in what you know about your employer into consideration here. Maybe you work for actual lizards that are happy to drop employees at a moment's notice, in which case I'd argue they're doing you a favour).
At this point, as pointed out, you could quit on the spot; but I would take other points into consideration.
- How regularly are they asking this of you? Is it infrequent? (i.e. can you be swayed by any form of repayment?)
- Do they have a concept of Time Off In Lieu? Do you ever get to actually take it? Do you get to take it at a time that makes it worthwhile?
- Do they have a bonus that you feel makes up for the overtime worked?
- Are you at the start of your career and angling for a promotion?
In my opinion, none of these things completely make up for the overtime, but they ease the chaffing a bit. If they don't offer these things, suggest them. If they're not forthcoming, then it's time to quit.