But you have to pay for the ability, no sane company will give you the ability to download yours products from their servers for free. They are not charity.
My guess is that comming into agreement in "we will sell only some stuff through steam (read, valve will get money only from fraction of sales), but all will be able to download from steam (read, also those who didn't payed to valve will be able to use their services)"
is harder then
"only those who will buy through steam (paying valve), will be able to use steam servers for download)"
If ED didn't come to agreement in the later, then its naivne to think, that they would come into agreement in the former.