[Ref in PA]

A1 drives to the basket. B1, defending near the free throw line, sticks his leg out slightly, slowing A1 a little. A1 continues and clearly beats B1 and has an easy scoring opportunity. A1 scores a layup.

If you call a foul on B1, A does not get the basket and takes the ball oob. Yes, B1 will have a foul in the books. Most of the coaches I know would not gripe about the foul, but would have preferred the points.

If B1's action allows B1 to retain guarding position or allows B2 to get into position to defend A1, then that exact same action should be called a foul. I think these scenarios depict why we are asked to judge advantage/disadvantage.