I agree. It's a BS decision. They have never had Plus discounted products tied to your PS+ account, so it's not OK for them to start, now. Especially when the discount is a measly $10 off a $60 game. Maybe if they were only charging you $10 or $15 to upgrade, but DEFINITELY NOT when they are charging you $50.
Having said that, I fully expect Sony to do the right thing and reverse they're decision if plenty of people make themselves heard.