Employment status is subject to some very vague laws in my opinion.
Technically they are engaged as self employed contractors by each track they ride at, they have to provide their own equipment and employ their own assistants if they so wish. They can ride throughout the EU for as many people as will contract them. Some riders are set up as limited companies and/or VAT registered. I think there is a strong argument for them being self employed.
Where it gets more tricky is the asset system but since (AFAIK) no rider has been stopped from earning a living as a result of the asset system I am not sure what legal basis there could be for challenging it. It has been used to create value within the sport.