Well yea, if you buy any digital game anywhere but GoG, I don't know any others that have policies like theirs, you're basically just renting it, you dont buy the game you just buy the license. GoG has a system set up to where you actually own the game, and if they went out of business tomorrow, you could still play your games fine.

Let's say you happen to buy a game or dlc off of a second-hand site and redeem it on steam, origin, or whatever launcher. You use it, and it works, and you play for a while, no problem. Well, if that key you bought is flagged for whatever reason, and the publishers of said game can decide to revoke the license you bought, and that's a wrap. Because that's all you're buying, even straight from steam or anywhere else - just a license. I used the second-hand website as an example because it does happen there. I haven't really heard anything happening on legit purchases straight from a vendor, but the legal wording is all there, and what I described is possible.