It's not like the receiver cut to the left or cut to the right and then got mauled. He stopped, turned, and tried to catch a football in the defensive backs running lane, which hit the DB in the back.
If you and I are running full speed together and I stop right in front of you (after the pass was thrown, mind you, on a timing based play), what are you supposed to do about it?
The receiver knows the play and the timing of the play. The DB doesn't. He can't be expected to know a ball is coming his way when the receiver hasn't even turned towards it. The pass makes all the difference here. It hit the DB. It was a bad pass. You're sandwiching the defensive player between a receiver and a pass on a timing based play. Totally unfair in my opinion.