That would be because the BBC would have a duty of care to those invited on to the premises and if it was shown they had allowed a young person to have contact with someone about whom they had reasonable grounds to suspect they were abusing young people, they could be held culpable.

Much like a person suspecting their dog was prone to biting, inviting children to play with it in their garden.

How far that duty of care would extend would be up to a judge to decide.