I've seen similar submissions allowed here in the past and my argument for keeping them would be that many of these third-party websites offer not only Steam-accepted keys, but some games that are only able to be activated on Steam. Where the key comes from shouldn't make any difference.

In my opinion the source of the sale doesn't make a difference as long as the game can cooperate with Steam or Steamworks in some way. I think if we restrict submissions too specifically we create too much fragmentation with game deals it makes it harder for people to get knowledgeable insight into which games work on what service because they will feel like their question is unwelcome in some places.