The 45-degree rule is hidden in their definitions. Here is that VNEA discussion:VNEA rules state that you must strike the cue ball at a 45degree ....
7. PUSH SHOT FOULS: It is a foul if the cue ball is pushed by the cue tip, with contact being maintained for more than the momentary time commensurate with a stroked shot. (Such shots are usually referred to as push shots.) With a cue ball and object ball frozen, shooting the shot from any angle other than at least 45 degrees above, or at least 45 degrees to the right or at least 45 degrees to the left of center from the straight line of the frozen balls is a foul and must be so called.
Note that this only applies to frozen balls, and is therefore completely goofy and broken. The 45-degree rule changes little about the hit for frozen balls. It can help avoid fouls on close-but-not-touching balls but doesn't always get you out of a double hit.
In fact if I want to play a push shot -- which is a shot that maintains contact much longer than usual -- I will jack up to 45 or so.