Because the current behaviour is like demanding everyone publish retail games trough EA or UBI soft and otherwise refuse to buy their game. Or if we take the movie industry as an example, it would be like refusing to see a movie unless it's from Warner Bros. The very fact steam is only handy as a monopoly should make one wary of supporting it, after all it's hypercritical to bash one company for doing something you support another company for.
Besides with the PC gaming market being as small as it is I laud any measure that see's the developer get 100% profit from their sales rather then fork over 30% to another company. Also unlike steam with origin you download it install it and don't have to start origin every time you want to start the game.
But maybe a compromise would be that the game costs 30% or more on steam so EA gets the same amount of money and steam lovers pay a bit extra for the convenience.