Answer by Rory Young:
When Zimbabwe first began de-horning rhinos they found the poachers still shot them.
The reason for this is that If they left them alive then they might end up wasting their time tracking a hornless animal at a later date.
Poachers will shoot a rhino for a little bit of regrowth because the horn is so valuable. The only way for de-horning to work is for all the animals in a wildlife area to be de-horned and for the horns to be cut off again when they grow back.