This is a common question which causes some confusion.
Although qualified to fly a type, the group or lessor wants some kind of "check" before letting you loose in their plane. Rather than pay for an instructor, they get a pilot experienced on type to carry out the training/checking.
If they were simply demonstrating systems and procedures to the new pilot, that might be ok, the problem I think is when they are supervising the new pilot flying.
If you, are PIC, but not an instructor, are you permitted to let someone else manipulate the controls? even if they are qualified, without passing "command" to them?
Are you trained to take control should the landing go wrong?
If there was a landing incident, there could be questions asked as to who was in command and responsible for the damage.
I believe that this type of "checking" of a qualified pilot (when a condition of hire etc) needs to be carried out by an instructor to be legal?