Except those are grey market keys and Valve can just take the game away from you once they detect it.

It's a major part of the lawsuit itself.

I did not set out with the goal of suing Valve, but I have personally experienced the conduct described in the complaint. When new video game stores were opening that charged much lower commissions than Valve, I decided that I would provide my game "Overgrowth" at a lower price to take advantage of the lower commission rates. I intended to write a blog post about the results.

But when I asked Valve about this plan, they replied that they would remove Overgrowth from Steam if I allowed it to be sold at a lower price anywhere, even from my own website without Steam keys and without Steam’s DRM. This would make it impossible for me, or any game developer, to determine whether or not Steam is earning their commission. I believe that other developers who charged lower prices on other stores have been contacted by Valve, telling them that their games will be removed from Steam if they did not raise their prices on competing stores.

http://blog.wolfire.com/2021/05/Regarding-the-Valve-class-action

So before saying they are wrong, see what happens in the lawsuit alleging just that

I've joined one of the Steam Dev Days conference in Seattle. It's around time where people was still doing things like cross buy etc. (so buying the game on website unlocks both steam, dev's own drm free version, maybe even console version.) I do not know if any of the actual developer term is updated after that time, but during the conference, one dev asked question exactly like this, can he sell his version without the 30% cut from valve if it does not going through Steam while giving away the steam key for free. The answer is no.

During the time it was explained that if you sell on different platform, that gives better sale %, steam can also impost that sale % on it's platform. At the time EGS was still not a thing but people asked about can they have different price on different platform, I think the answer is also no or not recommended, as they can request you to match say the base price of itch.io but they don't mind if that sale and software never use anything from steam. They specifically mention if any steam feature, like invite steam friend is used, then no, even if your game are not downloaded or use any steam distribution feature.

I find your 30% cut being less than adequate a very opinionated response. Steam gets to double dip repeatedly with anything that gets sold on their platform. Got a steam key? Now be prepared to always open their app which is defaulted to land on the store page and give you multiple pop ups when logging in. We also harvest your data for "advertising purposes" and you will need to constantly keep our client running if you wish to enjoy your game.

Imagine you had to go into the IOS or google play store every time you wanted to open an app and it started with suggestions for other apps you could use instead. If you wanted to use your app account on another device, even a pc, you need to download the playstore and link your mobile account to it just to then login to the app. Everyone would be shitting bricks.