While you all scream about these pay to fly schemes are degrading conditions, the main thing which makes these schemes atttactive to candidates is the fact that FI wages are awful and the average turboprop job has a three year bond with a salary of about 22000 a year. A 320 or 737 FO with 500 hours on type on the whole will be earning a decent salary. So for some individuals the pay cut you would take to instruct full time in order to get a tp job (paying terrible wages with a long term bond) means that paying for a rating and line training is actually the more financially viable option.
I don't necessarily support these pay to fly shemes, but also consider that if you go down the FI route and do the IRI (removal of no applied instruments restriction) and MEI upgrades you can easily spend 15000k on FI ratings (6000 for the FI (R), 30 hours p1 MEP time, 5000 (hour build in the US rates), IRI upgrade 1500, MEI upgrade 2000), then the gap between a rating and the FI route starts to narrow.