While there may be a slight bump in overall averages, I would suggest that it isn't simply due to advantageous rules for offenses. I think there are a lot of elements that go into these better rated quarterbacks.
I think one of the biggest reasons why QB ratings are up is a bit misleading. And that is the use of short, high percentage throws. QBs throw more passes, and shorter routes. Resulting in less interceptions, higher completion rating, and more yards. Which tends to stack QB ratings.
The best sign of a good quarterback is their completion rating in comparison with their yards per attempt and touchdowns per attempt.
Tony Romo is 6th in completion percentage in nfl history, 6th in yards per attempt in nfl history, 27th in touchdown percentage in nfl history.
All of which he has done without a stellar offensive line.