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You mean P2F 'line training'.
Paying for your TR is also P2F. As is paying for your MCC/JOC.
The airlines started this ... pilots followed.
Although I don't like it, i can understand why someone who has invested in a fATPL and hasn't had an interview for months or years, opts to pay for line training.
If I ever end up in that situation I will have to consider it, but it would be a last option.