This is an error fans make all the time watching american football. We are so used to slow motion replays that we forget what can and can't be seen in real time. As Twi said watching full speed it's not clear that the ball touched the net, even with the high quality of T2 video. If it was so obvious how did the umpires fail to see it live? And it's not like Ito argued the call. She looked to the umpires to decide and abided by their decision. If that causes you to lose all respect for an athlete then you have unreachably high standards of human perfectionism and I am very happy you are not my teacher, boss, or spouse.
Side note to NL and Sali. Feel free to stfu about Ito hiding serves anytime now. They all hide serves, and many of her opponents are also boosting. If she is better at serving than the others, including hiding the contact, that reflects massive practice discipline. Slating her for it when all her opponents are cheating in the same way or even more is like complaining about doping in cycling - true but irrelevant.
The playing field is level, only with a set of rules that diverges from the official book. This is common in most sports. Look at players diving in (un-american) football. Competitors have to adapt to the world as it is. That harsh and absolute nature is what makes sport so compelling. To complain that it lacks abstract perfection is to miss the point entirely.