Why do carriers require me to pay them a holdback just so I can work for them?
The holdback is a kind of insurance policy the carrier demands so that any charges you incur on the company's account can be recovered when you leave the carrier. Some carriers pay interest to the owner operator on the money held in escrow, others don't. There is currently no law requiring them to do so, so don't expect it. Ask for it!
Those funds held by the carrier aren't necessarily protected from the carrier's creditors in the event of bankruptcy either. If your holdback is retained by the carrier in its current account or even a interest bearing account, the money technically belongs to the carrier. Request that the carrier provides you some form of countersignatory authority on your money, otherwise you may never see it again.