Circular logic, no? Devs have to kneecap themselves by limiting their reach to stores with 5% cumulative market share or accept everything Valve wants. Take a look at this and see what happens when a big publisher goes against them:

gamedeveloper.com/business/-i-…

EA has issued a response to the game's removal, saying that it was "not an EA decision or the result of any action by EA," saying instead that the game was removed because an agreement that developer Crytek made with "another download service" violates an unspecified rule Steam has for its distribution partners. Valve has not responded to requests by Gamasutra for clarification. An EA spokesperson provided this statement to Gamasutra: "It’s unfortunate that Steam has removed Crysis II from their service. This was not an EA decision or the result of any action by EA. Steam has imposed a set of business terms for developers hoping to sell content on that service – many of which are not imposed by other online game services. Unfortunately, Crytek has an agreement with another download service which violates the new rules from Steam and resulted in its expulsion of Crysis II from Steam. Crysis II continues to be available on several other download services including Amazon, GameStop and Origin.com."]

They are not required to disseminate steam keys outside of steam. They do so because the generation of steam keys is free and they can keep 100% of the profits of using those steam keys. In exchange for the key being free they only ask that if the keys generated from steam are the same price both on steam's storefront and elsewhere.

If I am the manufacturer of a widget that you design, and I provide that widget to you to sell while footing the bill for the manufacturing of that product and only ask for 30% of the profits when I sell that widget in my store, but ask for none of the profits from selling the widget I manufactured when you sell it in other stores like Walmart, so long as the price of the widget remains the same on my store as it does in that other store, that's pretty much an industry standard.

If you as the designer wish to have someone else manufacturer your widget design and then sell that manufactured widget at other storefronts then I have no control over the price as well I shouldn't because that has nothing to do with me unless I specifically request exclusivity (which is similar to what epic does). <-- It has been alleged that Valve doesn't have or include this clause as part of their agreement, but they are trying to strong arm this developer into exclusivity in pricing anyway. There has as yet been no definitive proof that this is true and out of thousands of developers, literally like 3 are alleging this is true.

So there are two different things that are being alleged here and we don't have proof for either of them.

We have no definitive proof that they are strong arming any company/developer into price parity for game keys that Steam/Valve does not generate. If they were doing this, that would be an anti-competitve practice, but again , where's the proof?

And we have no definitive proof that the 30% cut of sales that Valve/Steam take for games sold on their store front are an egregious cost considering the competition and how similar their service fees are. In fact, the fact that one or two other store fronts take a smaller commission of sales is notable here because if Valve were a legal monopoly this is shown to be competition forcing market correction. Except that it's not doing so because other companies have basically refused to lower their commission cut. There is no definitive proof that Valve is colluding with other companies in the industry to keep that cut high. That would be an anticompetitive practice. I am not arguing that it's not (just that we have nothing substantiate the claim).