Technically no-one is being forced but network effects mean that not publishing on steam is essentially suicide for any dev without a huge marketing budget to overcome those network effects. This means that Steam is in a position where they can demand feeds in excess of what they could in a healthy competitive market.
Is there anything preventing a healthy competetive market for game stores on PC? Does Steam/Valve engage in behaviour that limits competition? Alternatives do exist - itch.io, GOG, Epic Game Store, and just putting an .exe file on your website.
If the market is not being artificially suppressed (I don't believe it is), and developer still think it's financially advantageous to pay Steam's fees, doesn't that indicate the fee is "correct"?
Another way to word that, is that the benefit that Steam provides through their distribution/passive marketing service is so large that it justifies the 30% price tag. If it did not, then developers would go find another service. It's basic economics.
