There's probably multiple reasons as to why the punishment is what it is in this case.

The obvious one's being the speed, conditions and surroundings. What probably didn't help was the fact that these two individuals were racing and had the wrong attitude following the incident.

Unfortunately we do not know all the facts and therefore we are unable to comment on the punishment in relation to the crime, there may have been numerous totting up of offences whilst the pursuit was taking place! We only know of the weaving in and out of traffic, we do not know what effect these actions had on other motorists or how many near misses there could have been.