I just experienced this looking for a hotel on friday night, a rate with no addons including breakfast at HGI Brindleyplace Brum is £41 on the summer sale rate for one person and £48 for two people despite being logged in as a diamond guest already.
I don't want to get reactionary but if you think that every diamond member is being overcharged everytime they don't think about this and book a room for 2 then the amount of money across all diamond members in a years would be in the millions easily.
Is there a way to restrospectively claim this amount from previous stays?