Many years ago I worked in Border Force (Customs as it was then known). When someone presents with a visa the details of that visa are not necessarily written on it. Those players were not the subject of public commentary in the day leading up to their arrival, so nothing flags them for particular attention. The border force officer would have looked at their visa, asked them to show their exemption, then on viewing that let them through. Djokovic drew attention to himself prior to entry. that meant the authorities were looking to the detail of his application. Turning their attention to the detail revealed not everything was in order. That provided the pretext in the current political climate on which to exclude him.