I'm guessing the seats you are seeing are preferred seats which would only be available if you were travelling with them. It would be helpful if you would provide more details about the seats you are seeing, as at this point, all we can do is speculate. The reality is that using your miles to book seats for others does not magically confer your status upon them and grant them access to preferred seats. You might find an agent willing to do so, but they are not obligated, so it doesn't really help to get into an argument with them about it. I'd suggest trying @DeltaAssist and see if they would do it. Remember to ask nicely, and indicate that you understand they are doing you a favor.