I feel compelled to add too - most folks know that the socially graceful thing to do is NOT to say back, "Why, it's not your fault". People who do make a choice to shift the situation to awkward as well, so shouldn't be completely absolved of responsibility there.
"I'm sorry" is a quick shorthand for the extended version, and we all know that. I think too as women we tend to apologize a lot, don't know why we do that, but we do.
The socially appropriate response is simply "Thanks".