Here`s another way of putting it;
Company A hires and types you, and you get paid from day1 of groundschool. They have thus invested in you because they believe in your qualities and devotion, and hopefully loyalty to them, and take care of their new investment.
Company B bonds you for say 3 years. They also want to protect their investment, and wish to have the pilot onboard for the duration of the bonding at least. If the pilot chooses to leave, then there`s a check to be paid, as agreed upon before accepting the job.
Company C only accepts you if you come fully rated. They most likely could care less if they need to sack you with very short notice, or if you bust a sim-ride or equivalent, and won`t give you a second chance. Unions have been known to be illegal in such companies.
Can anybody tell me why they would want to work for C if there are any other options?
As Oluf says, B is ok. They are protecting their investment, since they`ve paid for your training, but perhaps lack top notch T&Cs. They are also aware of this.