The ATP and WTA are autonomous organizations with the authority to determine their own regulations for their respective circuits (which is why on-court coaching is not permitted at slams because they are not WTA events). They are not required to consult with one another. The ATP does not allow on-court coaching since they never agreed on a regulation that would enable it. The WTA allows coaches only if they register with the relevant body (which in this case is the WTA). In short, there is no legal requirement that would prevent tennis' governing bodies from allowing on-court coaching. However, neither of them does so currently. Tennis has long been known for its use of visual aids during matches. Coaches are responsible for ensuring that their players are well fed, have access to water throughout the match, and are able to rest properly between points. As such, coaches often work with their players to ensure they are receiving adequate nutrition and relaxation before important games/matches. However, coaches are also responsible for persuading their players to change their behavior on court. This includes telling their players when they are standing too close to the net, where to position themselves on court, and giving advice about their opponents' strengths and weaknesses. Although guidance is available online and from other coaches who want to help but aren't allowed to do so directly on court, tennis' governing bodies take a strict stance against coaching on court.