That's simply the most well known example. Blizzard had a real money auction house in DIII which they got a cut of profits from virtual item sales so they clearly understand items have value. PLEX in eve online directly links real world money with their in world currency etc.
I could go on, but saying it's virtual is not necessarily enough to protect these companies.
IIRC, Blizzard doesn't profit from that directly and explicitly prohibits it in their ToS.