You may know government of various countries consider some of the areas as a No-fly zone where the aircrafts are forbidden to fly. Taj Mahal also comes under a No-fly zone. On 15 June 2006, a 7.4 km radius around the monument is taken as a no-go area because of the security purposes.
As we know, Taj Mahal is the biggest attraction and is one of the world heritage sites. Airplanes cause the air and sound pollution as well. To protect the monument from environmental crisis, aircrafts are not allowed. Nowadays, due to higher air pollution it's marble is turning yellow. Hence, it's protection is important.
Tourists also need peace there. They want to see the beauty rather than engaged in pollution and annoying sounds. Taj Mahal is a part of economy for India. Government earns billions of money every year. Therefore, all the disturbances are restricted to allow more and more tourists to visit.