in my limited experience, many aftermarket rearsets (for many bikes) do not utilise return springs. Possibly due to the lighter weight of the aftermarket lever.
On the inside of the rearset is a disc that is drilled off centre. When the unit is fitted you can set this to prevent the brake lever moving when not in use.
The master cylinder itself is enough to return the lever to neutral position.
i rarely, if ever, use the rearbrake lever; but haven't had any issues whatsoever hope that helps