Steam is a service that costs money to keep running - lot's of money actually in their scale. When you sell a Steam key outside of Steam, they don't get their cut which goes toward running costs and whatnot. It doesn't of course matter if it's just some random few keys but if almost all devs started to do that, it could cause some serious funding problems to Valve. That could then lead to reduced service levels of Steam and that would hurt their customers - the players - the most.

So while it's not a big problem currently, it could be if it wasn't prevented properly in contractual level. People who think that is an unfair clause don't probably understand what it actually takes to run a service like Steam or they are straight competitors trying to run them out of business in any way imaginable.

E: And actually if Steam still allows selling the Steam keys in external services but only requires the price to match the price in Steam, it's already a quite charitable policy. I guess they count on not too many people buying the key externally for the same price than in Steam store.