The engine is open source. The game is not.
Return to Castle Wolfenstein runs on id Tech 3, and that engine was open-sourced, along with the engines for everything else id made in that era. Browser builds exist because of that release.
The same rule as Quake applies. Free engine code, commercial game content. The browser builds need the real data files from a copy you own.
Bethesda sells the game cheaply and it still runs on modern Windows.