Yeah, that was the issue. One of the big things that entices people to the alpha is more content. If you can just add more content through modding the demo, there kinda isn't a point.
That's because RoR was made by people in their spare time, and it's not the most advanced thing out there. BeamNG is extremely advanced, and making it is the developers' only job. If they didn't make it cost money, where would they get money for food and such, and also for continuing development?
Also, please make sure to check the date of threads before posting (If the only thread on the topic you can find is really old, I think it's a lot better just to make a new one.)