Where is the line drawn in what counts as an "actual" obligation? Just because it's their job that means they're obligated to? That seems awfully arbitrary. A fireman or police officer, or really anyone for that matter, can just walk away from any situation they don't wan to deal with at any given time. Sure there will be repercussions, but realistically there's nothing saying that firefighters must put out your fire. It's not like a fireman will be put to death if he doesn't help.

The reason firefighters help is because there's an expectation set on them that they will. They signed up to be firefighters so they should be prepared to deal with fires. But nothing's stopping them from just walking away from one, really. There's no omnipresent force that's preventing a firefighter from leaving.

And my point is - since we have that expectation that they will help, it makes it a lot more morally wrong if they were to just walk away from a burning building. Realistically they completely could just walk away, because there really is no obligation, but it would be very inappropriate for a firefighter to do that for obvious reasons.

And this can be applied in a similar way to social scenarios. If you go on multiple dates with someone or talk to them regularly for an extended period of time - there's an expectation of mutual respect and trust growing between the two of you. And when you do something inconsiderate like completely ignore them, it really can't be morally justified by "well I don't an obligation to tell him/her anything." Sure you don't, but it's still wrong to do for obvious reasons, saying "I don't owe them anything" doesn't make it any less shitty.