For the uninitiated, why is it easier with a license?
The Spitfire doesn't have a standard certificate of airworthiness, so strictly isn't allowed to be used for passenger carrying on a commercial basis.
There is however an exemption that allows remunerated training to be conducted otherwise there would be no route for people to obtain the necessary training on type.
The fact that this 'training' is only used for a one off experience doesn't alter it's legal validity.
This exemption doesn't extend to ab-initio training of an un-licensed pilot as there are more appropriate certificated aircraft that are suitable for that purpose.
There are some routes for people to offer privately arranged passenger flights whereby they pay only a fair proportion of the operating cost. Some owners are willing to do this as it reduces their cost of keeping it flying and there is also a quite generous spirit amongst many of them in wanting to give people the opportunity to experience it. These private flights are essentially not advertised so that they are not seen to be offering it on a commercial basis.