The Dimaond Status Amenity is the same as the Platinum Status Amenity.
So I mostly stay at the same few BW properties over and over, I see the pattern: If the clerk checking me in is paying attention, I get the welcome bag. If the clerk checking me is not paying attention, and doesn't realize I should get something, I get nothing. In other words, the hotel awards 250 points if they don't want to give you a welcome bag, but if the clerk didn't notice your status or didn't realize you're supposed to get an amenity or points because of your status, then how is the hotel supposed to know that?
It therefore seems to me that they would have to change the check-in computer system at all their (individually managed & operated) hotels to automatically give 250 points if the clerk doesn't actively click something to say "I gave the guest the welcome amenity instead".
But that change would liekly lead to the reverse problem, of people getting both the welcome bag and the 250 points, which you as the platinum/diamond member might like, the BWR woudln't like.
So how likely are they to implement such a change?