[bendoidic]

All the art and sound assets are loaded from a local copy of C&C that the user must purchase on their own before running OpenRA (specifically the "C&C Remastered Collection" on Steam or Origin). OpenRA uses the assets from that local install, so it's all legal! Stratagus does the same thing with Warcraft assets.

https://github.com/OpenRA/TiberianDawnHD#asset-installation

[xsmasher]

OpenRA will download the campaign and graphics assets for you (not sounds and movies). No purchase is required.