muntdefems: Guess IGN/Humble feel they don't have enough linked Steam accounts yet...
MarkoH01: I wonder if they gain something in forcing people to link to their steam account (which technically would not be necessary).
I was mainly joking, though you never know...
I imagine they do this to avoid key hoarders to accumulate hundreds or thousands of keys and then proceed to sell them some time later. That is, assuming these free games get immediately activated in your Steam account once you "purchase" them on Humble. If they don't and you simply get a Steam code to do as you please, then I don't see the point, really...