It's a human rights issue. The US doesn't have a very good record. The most usual scenario in which this comes up is in extradition cases where the person invol

It's a human rights issue. The US doesn't have a very good record. The most usual scenario in which this comes up is in extradition cases where the person involved could face the death penalty (of life imprisonment I think too). It wouldn't make sense for the UK (where there is no death penalty) to extradite a person to somewhere they no longer have that right to life. This is just a natural extension of that policy.