Think about the principle this way: If there's a long pass play where B intercepts the ball in the EZ and takes a knee, that's normally a touchback. But, if there is a penalty on B during that down, such as DPI or Roughing the passer, we are not going to go to the 20, mark off the penalty against B and give them the ball. We're going back to basic spot, which in that case would be previous spot, mark off the penalty against B, and A keeps the ball. This situation is exactly the same as far as the principle goes.