You give them a cut of the turnover on their site(steam). Important distinction. A developer can generate steam keys for free and sell them elswhere, as long as the price is the same as on steam.
As I understand it the issue is actually that people weren't allowed to sell their game for less on other platforms, but they weren't necessarily trying to sell Steam keys.
Yes it is controversial, cause Steam pretty much has a monopoly on the PC market. Yes there are other options, but your games just won't sell as well cause less people use them. So deciding not to pay Steam's 30% cut isn't very viable.