The Courts are not doing this, they have convicted the registered keeper of the vehicle as laid down in the Road Traffic Act.
They have not levied a conviction for the actual speeding offence.
The following is from S172 of the Road Traffic Act 1988
(2)Where the driver of a vehicle is alleged to be guilty of an offence to which this section applies—
(a)the person keeping the vehicle shall give such information as to the identity of the driver as he may be required to give by or on...