The rule itself is bullshit!! Why can a RB reach over the goal line, fumble, and be awarded the TD for breaking the plane, yet a WR can make the catch, get both feet down, get tackled, and be forced to hold on to the ball through the process??
Because a RB is already established as a runner when they cross the goal line. The instant the tip of the ball touches the line it is a TD no matter what else happens. When a player is already in the end zone and catches the ball they have to complete the process of the catch in order for it to be a touchdown. The pass catcher in the end zone has not yet established themselves as a runner. Very similar to when someone catches the ball in the regular field, gets hit before making a football move, the ball comes out, and it is ruled incomplete instead of a fumble. If a wide receiver catches the ball at the 5 yard line and runs it in they are treated the same as a RB because they are established as a runner.
They grey area is when the catch is completed. When you are in the end zone it is completing the process because you don't need to be a runner but you need to maintain control through the entire process the same way that you do in the regular field of play.
Now, this is of course, all subjective to what the Zebras think they saw and which review specialist is making the call in the booth, and what kind of day they have had as well. which seems to change almost weekly.
An example in the DETvs CHI game the guy was not established as a runner when the ball broke the plane and was knocked out should have been ruled an interception because he did not complete the process. I am still not sure how they overturned that one. However the Dez bryant catch last night was the exact opposite had control of the ball, knee touched the ground, split second later opposing player punched it out, counts as a TD because he completed the process to the ground.