The system Smokie describes is also used at my company (may be the same one reading some of his/her other posts)
At the start of the bonding agreement a sum equivalent to the loan is paid into an escrow account which is released to the company if you leave or at the end of the bonding period, or to the bank to pay of the loan should the company go bust. This protects the employee if the company goes under.
Assuming you have no intention of leaving it is a fair system (as is any bond) - but has an advantage if you are a poorly paid turboprop F/O as if you are offered a position on a heavy jet you stand to gain about £15K p.a. out of which you can easily pay £300 per month for a loan. ie the bond doesn't actually stop anyone leaving if the company pay scales are low enough - as they maybe .