There's an interesting nuance in the phrase "a certain moral obligation." In my understanding, it's a fairly weak statement [1], somewhat akin to "People will think more highly of you if you do this thing, and less highly if you don't."

Contrast this with "they have a moral obligation." To me that is a much stronger statement: "People will think you are behaving in an immoral way if you do not do this thing."

It's funny how adding the word "certain" to the phrase actually makes it a less certain, gentler way of putting things - quite the opposite of the literal meaning of the word.

Put another way, I could imagine a law being passed to enforce "a moral obligation". But it seems much less likely to have a law enforcing something described as "a certain moral obligation."

[1] I mean "weak" in the sense of not being a strong claim of obligation, not "weak" in the sense that the phrase is a poor way to say something.

Right, I just meant in a "pay it forward" or "the decent thing to do" sense.

If you're referring to software license terms, you're making a legal, not a moral argument.