I mean, it makes sense since the bail bondsmen have business licenses and likely some form of insurance and credentialing with the courts to cover the differences. All the court really cares about is if someone is liable/responsible/trustworthy.
Multiple states have no insurance requirements for either the writing of the bond, or the recovery of a fugitive, and more states beyond that have no credentialing or training or licensing requirements for recovery agents.
That's still a better deal for the bondsman, but it's also reasonable. If you don't show up for court, it doesn't seem likely that you'll show up to deposit the other 70%... But if you don't show up for court, the bondsman probably hasn't left, and they will be available to pay the rest of the bail amount.
Of course, most bond contracts say that the bondsman can begin civil recovery against you for that full amount too.