It's not the unions' fault it's the wannabes - complicit with the schools who are quite happy to feed them BS and take their money.
There was a time about a decade ago when the lo-cos did in fact hire experienced pilots, turboprop guys and FI's, and bond them, just like everyone else. This buy-a-type-rating thing was available but pretty uncommon. It only took a few people in the early '00s who managed to jump the experienced pilot queue by buying those ratings and those hours - and then surprise surprise, the schools realized they were sitting on a gold mine and started aggressively marketing the type rating as part of all basic training.
Pretty quickly after that, the accountants in the lo-cos noticed this and realized they could get into bed with the schools, and collectively make a fat profit. All the other ideas, charging for interview, charging for "line training", flowed from that. Now it's an integral part of their business model and it's not going ANYWHERE.
But I really don't believe the idea didn't come from the airlines initially, it came from the few
rich greedy short sighted selfish wannabes who did it first. The damage was done so quickly by the mid '00s, I really don't blame the unions for not seeing the danger until it was too late.