It is a loophole being exploited by a certain type of school/business and it is being used in what certainly was not 'the spirit of the law' to make a quick buck at the expense of safety and maintaining professional standards.
Unfortunately Joe public has no idea of this when he walks into the flying school. They paid for a gift voucher with a pilot, they were provided with a pilot, they have absolutely no idea of the difference between an enthusiastic PPL holder and a professional FI.
Waiting for the first incident where a PPL was letting a random stranger have ago, when they themselves have not been fully trained in how to deal with this situation.