I am tempted to think that this was the first widely adopted (and accepted) instance of augmented reality. Very cool in that context!
We have a bay in our lab with a to-scale football field where we can run indoor tests, but on the field it is computer vision that is doing it all. The cameras are instrumented to get attitude, zoom, and so on (we have to modify the lens to get the accuracy we need), and then of course you have to compensate for lens distortion. If you look closely, the "line" is not a line, but a curve. If we drew a line it would look stupid, and not reg well in most parts of the image.
I can also imagine MLB having similar concerns as the NFL when, for instance, they insisted on removing the line as the ball is being spotted. That is, if they are showing the strike zone and the ump's call seems inconsistent, then it wouldn't be a "good look" for the sport.