Well.... the truth is that they may have been convicts but were regarded more as troublemakers than criminals...
So, can you just imagine some "highbrow" British Lord (or whatever) saying to the effect,"We'll bloody well show you! We're going to send you to a place where the sun shines year 'round and it is also very warm!.... You're going to be sorry!".... and when the "prison" sentences were up, the authorities announced that "You may now return to England!".... AND NOBODY WENT BACK TO ENGLAND!
and that is really why, to this day, Australians refer to the English as POME's (Prisoners Of Mother England)... as the REAL prisoners were NOT on the list to be sent to where it was/is sunny and warm!....
I gotta LOVE my (in Bert Munro's words) "fellow Loyal Subjects Of The Queen!