Why not? / safety / braking.
With motors grounded there will be substantial drag if the motor shafts are turned, due to their acting like shorted alternators. This is unlikely to be an intended effect but exists.
If the motor is grounded when not used there is minimum chance of a fault elsewhere allowing the motor to operate.
If a contact is floating it has the potential to be shorted to something which can cause a problem. If grounded then such a short would cause the PTC to overheat and no action to occur.
In practice, it probably makes extremely little difference in most cases. In the cases where it does make a difference, the difference is a positive one from a safety point of view.