Yes but like any business should have some liability to claim from? I think it is just a really cheats way out of not paying out for their own lack of care.
For example my uncle is a carpet cleaner...are you saying if he offers an insurance of £1.50 per carpet being cleaned, the client refuses but he cleans the carpet anyway, and by doing so the carpets are damaged due to him not giving a damn about his work but because the client didn't pay for the insurance he gets away scott free?
It's not quite so simple as that. Most couriers (even RM) will give you free of charge insurance, however only up to a certain limit.
Look at it the other way - should the courier accept infinite liability; e.g. if you were to send a hypothetical object worth £200bn, should the courier accept full liability for that £200bn? (arguably yes, but mistakes happen, hence why we pay for insurance(or not

))
Their own insurance probably has certain stipulations about number/value of items they handle.
To use your carpet cleaner analogy - if your uncle turns up at a client's house, and they have carpet which is £2000/m^2, and your uncle's insurance policy only covers him up to £200/m^2, should he:
a) tell the customer he can't do it and leave
b) tell the customer he can do it, but they'll have to pay a extra so he can upgrade his insurance cover for the job, or take responsibility if the carpet gets damaged?
The alternative is that your uncle pays for insurance to cover £2000/m^2 and charges ALL of his clients extra to cover the cost, even though only 1% of them actually require that level of cover.