They're a distribution mechanism. If you buy a Steam game you need Steam. Allowing developers to require Steam to play their game is not anticompetitive or in any way unethical.
They didn't force any developer who wanted to sell games on Steam to only sell games on Steam. That's what would be anticompetitive and abusing their market position. Games choosing to only distribute through Steam because there's no other storefront that wouldn't be a worse value if it was free isn't Steam doing something wrong.