This is making some steep assumptions based on a singular sentence discussing the beta's availability. The Elder Scrolls Online beta was ALSO Bethesda.net Launcher exclusive and its fully available on Steam today.
This is how Bethesda does betas and has always done betas for their software
Couldn't blame them or any other producer moving away from steam. As much as I rely on steam, it has become such mess trying to find a decent game on there. Any old can go on sale there and is rife with scams and half baked games.