I don't think there is any link between the two...
You have companies not charging for the selection process BUT where you will be bonded and companies where you have to pay for the selection process BUT where there is no bond...AND the one where you get both...
The main purpose of the bond is just to try to make sure that the pilots joining will , at least , stay for 3 years AND it seems to work.
When pilots will be ready to leave despite the fact that they have to pay a bond then the bonding will dissapear but as long as it is working and as long as we accept to sign it why should they stop doing it ?