I totally understand you're asking about moral obligation, not legal obligation, and I totally understand that those are different things. I also agree with your unspoken implication that we have no moral obligation to comply with an immoral law. :-)

It's a conditional blessing. They've provided a service subject to certain terms, which they've disclosed prior to you using the service. Your moral obligation is to either accept the terms as they've established them, or not use the service, or negotiate different terms.

If I say you can borrow my car if you have it back by 6pm, then you've got the same choice. It's not morally defensible to say, "well, he's not going to use the car, he doesn't *really* mean 6pm." Those were the terms, and you accepted them. Morally, it doesn't matter *why* I want my car back at 6pm -- it's my car so I make the rules.