I believe it is a simple leasing company / accountant argument.
If you lease day cabs it may cost less but they will struggle to remarket them.
Look at the Motability cars, they buy higher spec cars so as to be able to sell them on at 3 years with low mileage.
Hire cars used to be low spec with no extras, now the companies realise there is a market for reasonably priced used cars in the auctions.
I don’t see the weight issue with supermarket trolleys, only with bulk tippers or tankers