No they did it because that was the only way they would have been allowed to sell digital music downloads. I think you under estimate the control that the record labels and RIAA exerted on these digital stores including iTunes.
Just because iTunes was first doesn't mean they had the ability to buck what the record labels and RIAA wanted. Eventually they got big enough that the record labels and RIAA knew they had to adapt to this new digital world, but they continued to push DRM as part of that process. It was acutally the smaller independent labels that lead the move to non-DRM music.