Taxis and buses are exempt because it would be impossible to have the right seats for all children,
If a parent stopped a taxi and they had twin six month year old babies, a toddler, how would a taxi be able to carry the right seats for that combination, and adjust the seats to fit each child, not realistically possible,
As well as carrying the seats for the same person two years on, with twin two year olds and four and half year old.
If they made it that a taxi had to provide suitable seats they would have to have huge boots,
only certain seats fit certain cars,
the only way round it, would be for the car manufactures and the child car seat companies, were made to have an industry global standard seat, belt, arrangement, so parents could carry their own child car seats (which would be fun, for the above example of parent)
I doubt the car companies will agree to limit car design as that how they compete with each other.
or that every taxi to be a vehicle that all child car seats fit to, and parents carry their own at all times, or not use them,
no government is going to ever attempt to tack this, so they will for ever more be exempt.
taxi insurance companies, must of decided the small amount of pay outs were worth being completive for the taxi insurance trade.