Because it is a silly rule.
The team should lose the arrow if it fouls OR violates during the AP throw-in. Just my silly opinion.
The current rationale is that an AP throw-in ends when the ball is touched inbounds or the throwing team violates. Since the arrow is reversed when the throw-in ends, we have a change for a violation.
However a foul is not one of the ways listed that the AP throw-in ends. The best I can say is that the throw-in process is interrupted. Since the AP throw-in didn't end, the arrow is not reversed.
As I wrote above the rules committee should change this by adding "or fouls" to the end of 6-4-4.