AST for NCA/SNCO ATC candidates has almost certainly been introduced for standardisation of entry processes. There is potential for other airmen to complain that some airmen do not complete the same entry tests as are otherwise universal for airmen, ie AST. This could be a factor in any formal complaint about recruiting practices as well as creating an unnecessary whinge - there are enough valid ones already.
Conversely, I don't know what they did previously about NCA course failures, but without having previously taken AST tests, how are they offered re-trading?
Adding the AST requirement to NCA/SNCO ATC applications is very cheap to do, all the resources already exist, there is just a slight increase in demand/capacity, but that will be well within AFCO capacity to absorb.
If it reduces the number of NCA/SNCO ATC candidates, so what?