"Why on earth would you want to keep paying for them to be held on an island?"
Because this stupid govt signed security contracts with a business that has clauses in it that increase the value of the contract irrespective of the numbers of detainees. Even maximum security prisons are cheaper to run.
Put them in a community with requirements to report each week or wear a tracking device. We do that with convicted killers and sex offenders so why not a family of 4.