And now it's explained, though in an article that seems to
oddly place the blame: the citi e-bike allegedly lack a "power modulator" which according to the
Shimano manual for the roller brake is intended to prevent precisely the situations reported in the press:
You need to use the Shimano front INTER M brake body and the hub as a set (excluding BR-C6060). The hub of the Shimano front INTER M brake has a built-in power modulator. This system controls the braking force so that excessive force is not applied if the braking force reaches the specified value. If the front brake is applied too strongly in case the hub is not equipped with the power modulator, the wheel may lock and the bicycle may fall forward, and serious injury may result.
Or another explanation - it's basically a
clutch in the hub that limits the ultimate torque the brake can apply to the wheel
Presumably they aren't using the Shimano hub because they have a hub motor from a different source, so either there's no clutch to modulate braking, or if there is it doesn't work as well as it needs to.
(A little background reading reveals power modulators have their detractors who feel braking is less than a skilled rider could safely achieve without, but these are
share bikes)