This is how I always describe this when someone states "whole ball" has to be behind the line. We are talking about where the position of the ball is on the table. In this case behind the line or not.
Let's look at it this way, you shoot a ball in a pocket and it's hung up, with 1/3 of it over the hole. Is it IN? If someone plays by full ball has to be behind the string, that means the position of the ball is where ANY of the ball is not the base. Therefore if I hang a ball in a pocket like that against you, the ball is IN the hole since a part of it is over and we established that position of the ball is in the place where ANY of it is touching that space. Since 1/3 of the ball is over the whole, the position of the ball is in the hole.
Same exact reasoning, if position of the ball on the line is all or nothing, then position of the ball by the pocket is also all or nothing. You can't say it's one thing one way and another thing the other example.
Plus the base of the ball is easier to judge where it is on the table and that is where it's actually touching, rather than some imaginary line going up from the table to the edge of the equator of the ball.
And it's the official stance of the WPA to go by the base and every tournament and league I played in.