“There is a difference between “Why observe?” and “Why observe on this calendar date rather than the other one?””
while there is a difference, it isnt as great as you imply. and the questions are similar in that they are equally silly.
There is a response from R’ Miller that makes it rounds every independence day about putting up the flag on July 4th. Nobody (as far as I know) has a hava amina to commemorate independence day on its Hebrew date and raise a flag on 17 Tammuz.
This thread isnt about descendants observing Yahrtzeit. Obviously that would be on the date of the Yahrtzeit, and I dont think the OP was askign if family members should observe the yahrtzeit of 9/11 victims.
If there is a role for commemorating 9/11 (again outside of routine yahrtzeit) then it would be on the date it is commemorated. Thus the questions are quite linked