Xkcd rule of dating
The purpose of the app is to get dates, with the intent of a romantic relationship or sexual intercourse.
However, in the comic, Cueball is trying to use it to request assistance flying a plane instead.
I don't think it's an actual "law" since the entire concept is based on garbage and bullshit (you'd have to be in the cockpit AND within about 2 feet of the equipment in question in order to interfere with it.
This comic is similar to 1897: Self Driving, and as well as 582: Brakes, which also is about bad ways to get help in emergencies and other time-critical situations.
Note that the photo is at an angle, but the view out of the window shows the airplane to be in level flight.
Wi Fi can be enabled on some flights during the entire flight; in others it may be banned during takeoff and landing.
Even if he is either uploading the picture after the flight or using the in-flight internet service, he is still violating other, more serious laws (if he is a pilot, he may be liable for negligence, and if he is an ordinary passenger, God knows what he may have done...) The title text explains that Cueball's unwise method for getting help stems from astonishingly skewed priorities and no small amount of selfishness.
He claims to strongly dislike conversing over audio-only channels, and this dislike is apparently so overwhelming that he would rather jeopardize his life and that of any passengers on the plane, than put aside his own hang-ups.