Yes you are obligated to do such a thing, and if you don't comply you can be dragged to court where you could be sentenced to pay for copyright infringement.
Exactly: not complying, getting dragged to court, and paying is an option. You're not obligated to open the source, you can just suffer the consequences of copyright violation instead.
I'm not sure if you're agreeing with me or not, but if you aren't, then how does this not mean that you're obliged to open the source? You're obliged to follow the law, and if you don't you'll get punished for it. Am I misinterpreting the word obliged? Because that to me is a perfect usage of the word obliged. Correct me if I'm wrong but is everybody thinking I'm saying forced? Because I'm not. http://www.thefreedictionary.com/obliged