These cars are ONLY availale in the demo and that's how the demo works.
You started with a low-end car and in Sport mode there are events for Gr.X, Gr.4 cars too.
You do not own cars from other classes except your first car so they let you choose some cars in order to play in Sport mode.
Also, we can NOT buy cars from Brand Central in the demo (and that's the reason why you get free cars to test/rave with).
We just get a free car everyday.
When the game releases you will start with your first car and maybe the cars you have collected in the demo.
You will not get any free cars so you will need to buy them especially if you want to play in Sport mode, because Sport mode needs cars from specific classes.