That's true of any permanent-magnet DC motor. The term is "dynamic braking"; the braking force is proportional to the speed of the motor. If the motor isn't moving, braking torque is zero. The only remaining opposition to movement would be provided by the cogging force of the magnets, and the friction of the bearings and brushes (for a brushed motor).

We had the same problem when racing on a sloped parking lot. Using a LeMans-style start, with the cars against one wall and facing across the track, solved the problem. It also made it much more difficult to get a hole-shot!