Who decided that the Boeing 737 MAX planes that were airborne when the grounding was issued cannot enter and land in EU air space?
The operator's airworthiness department interprets the AD and informs the operations department and senior management.  Ops decide how to deal with airborne aircraft. 

The EASA AD actually had more generous continued flight provisions than the FAA issued AD.  It was in no way intended to prevent aircraft in flight from landing.  Airworthiness would immediately talk to operations.  Operations would decide what to do, though in most cases a few phone calls would be made to senior management.  If the aircraft can't make it back to a 'home' port, that pretty much dictates what is going to happen next (continue to destination).  In the subject case, a number of aircraft were airborne and capable of returning to a home port.  So you need to figure out what to do next - continue flying to your EU destination and get your passengers there with minimum inconvenience (with possible adverse publicity for not discontinuing the flight as soon as possible), or simply get the aircraft back to base so that you don't have it stuck somewhere needing maintenance (and reading between the lines, Ops and Airworthiness would be concerned that the recovery might become more complicated than the 3 flight cycle limit ferry suggests).  So better to get your pax down ASAP, at a port where you can easily carry out whatever rectification is needed, and importantly too, also one where you have more aircraft available to get your passengers to their destination without having to do positioning flights. 

In some cases, such as TUI, they elected to divert the aircraft to the nearest suitable port, regardless of destination or home port convenience.