revisiting payware/freeware dependencies

1) I'm still thoroughly confused about what assets I can use, and not require players to buy other routes in order to play mine.
In the RSC assets folder I see assets for Bath/Templcombe, Hagen Siegen, Cajon Pass, Sherman Hill. I have those routes and assets, because they come with Railworks/Train Simulator. Everyone else who owns Train Simulator has those routes/assets also.
So why can't I freely use those assets in building my route, with the knowledge that everyone has them?
2) I note that assets for Horsehoe Curve are also in my RSC folder. I do not own Horshoe Curve. Therefore, I understand that I cannot use those assets. (I guess).
In the RSC assets folder I see assets for Bath/Templcombe, Hagen Siegen, Cajon Pass, Sherman Hill. I have those routes and assets, because they come with Railworks/Train Simulator. Everyone else who owns Train Simulator has those routes/assets also.
So why can't I freely use those assets in building my route, with the knowledge that everyone has them?
2) I note that assets for Horsehoe Curve are also in my RSC folder. I do not own Horshoe Curve. Therefore, I understand that I cannot use those assets. (I guess).