I might be unpopular with other Drivers on here but here goes.
Working a rest day is additional to your normal salaried hours, if a Driver is unhappy with the financial incentives offered, they are perfectly entitled to not work those rest days and stick to their basic salary.
If the rest day rate offered is paid at normal flat rate and Drivers are still volunteering to work their rest days, they have decided that the rate on offer is satisfactory to them.
On the other hand, if the rest day rate offered is paid at normal flat rate but not enough Drivers are volunteering to work their rest days, surely that is a matter for the company to decide whether they should offer additional incentives to entice more Drivers to come in and cover jobs on their rest days?