To me, it seems to kind of point to belief in or support of "fate" or "destiny," that she was supposed to die there, then, and it would have been wrong to undo that.
I'm sure the parents would totally agree.
Mother: Why won't you bring her back?
Riker: It is our duty as Federation officers to save lives, not bring them back.
Mother: But people have been declared dead many times in the past and were brought back to life. Even Bones and Spock died and were brought back and no one said they should have remained dead.
Riker: Listen lady. Do you have any idea what this power can do? If I bring your daughter back, I will become so corrupted that I will declare everyone around me inferior and kill anyone who disagrees with me!
Mother: ....Were you always that kind of a person?
Riker: Me? No! I hate death! But the point is that your daughter is dead and it's best to leave it that way. Look on the bright side. Your daughter died to help build my character.
I wonder if there's any parent alive who, if given the chance would want their lost child brought back. No cost, no strings attached. Just some guy who got god like powers and wants to help.