I am not saying I necessarily approve of pay to fly, but it is quite easy to see why it has become so common.
I know of three turboprop operators in the UK that use to bond; all three now expect FOs to pay up front for the rating
Ok you say paying for a rating with a job offer is acceptable, but consider in this case each of these operators are exclusive in the type they operate and to stand a chance of getting an interview with these opeators will normally mean instructing (or some other GA activity) to get 700 to a 1000 hours.
(Companies in question are Eastern, West Atlantic (formerly Altantic Airlines), the third one will remain nameless, insider information gained in confidence).
So overall outlay to get a job on a rare turboprop used by no other operator; 7K for an instructors rating, a year of working on an instructor pay (15 to 25 pound per flight hour, may a small retainer if lucky), then fronting between 12 to 18 k for a rare tp rating.