A few things. First off this is how Steve Johnson has basically made a career in freelancing routes. Secondly, yes receivers have to make reads pre snap, same as the OL, RB, and QB. Yes sometimes WR's will run different routes based on coverage reads. But more commonly it will be smaller adjustments looking for the soft spots in the D. So depth of route changes based on the read, when to make your cuts, whether the WR comes inside or outside. So on a post, Chandler makes a decision on when to cut that route in based on what he sees from the Safety and the LB.
On a similar point to the OP, Rich Gannon was really hard on our receivers the other night for the exact same thing posted here. He said that they were a talented group but were making a lot of wrong reads and running a lot of poor routes given the situation. People here took it as Orton love. I didn't really get that at all, I thought he was hyper just hyper critical of the WR corps. By no means did it seem like a free pass for Orton.