I think it's a legal thing. There is no way legally UEFA (through La Liga) can ban a company (Barca) from hiring employees.
However, they can restrict a company from registering players for their specific competitions, both CL and La Liga. So, by restricting Barca from registering new employees, UEFA/La Liga are effectively circumventing European fair employment laws by saying "Well, you can hire an employee but that employee won't be allowed to participate in our competitions which just happen to be what your company does."