Just like other professions?
How many doctors pay to operate on you? How many tube drivers pay to carry passengers? How many solicitors pay you to take on your court case?
After you have qualified as a doctor, lawyer, bus driver, you are paid to do your job.
The way to stop this rubbish is to say no! If the airlines can't find the crews they will pay you to fly.
As for those who don't care if folk p2f as its not their money, well, if you are professional aviators then you should be concerned because it will affect your ts&cs very soon. If you are a passenger, you should expect the very best aircrew up front not a p2f cadet who is being ripped off and will be back on the dole queue in a couple of months with less airbus time than the min required for a real job.
As for those that claim p2f cadets are now getting commands - piffle and twaddle! Name some! I just met 5 a320 guys who are out of work because of p2f.
As for those who claim instructing is for low wage earners - well at least they earn! Actually they earn £43k in some uk flying schools.
PPRUNE and BALPA should be ashamed of themselves for allowing p2f advertising at all.