RSPCA breaks the law on releasing non-native species

In the UK, the grey squirrel is a non-native pest, which is a threat to the native red squirrel. Indeed, it is so much of a pest that if one is captured, it is illegal to release it.

So why did the RSPCA officer release this one? It should have been humanely dispatched. Surely the RSPCA Inspector should know the law?