You mean like they do on Origin, Steam, UPlay, GOG and on and on? Every publisher pays these fee's if they don't have their own service like EA.
You are confusing retail fees with platform holder royalties
retail fee = the cut gamestop, greenmangaming, amazon, the steam store (emphasis on store) take for selling a game on their storefront.
EA for example have their own storefront (origin) because then they don't have to pay any store fees and make more profit on their games. Same goes for blizzard (they don't sell on third party stores at all only on battle.net (and in retail ofc because not everyone buys DD)
platformholder fee is what the name suggests, it's a royalty fee you pay to the platformholder to be publish your game on their platform.
It's an additional fee on top of the retail cut ,EVERYONE who sells a game for a playstation console or on IOS (apple's mobile OS) pays that fee.
When blizzard sell diablo 3 for ps4 in gamestop, sony is still getting a cut of that.
Valve are not the platformholder of PC (pc has no platformholder) , you can sell a steamworks game in your own store or at retail or in another online store like greenmangaming or GOG and valve will NOT get a cut from the sale even though it activates on steam.
If valve were the platformholder on pc then they'd be able to prevent anyone from running an application on the pc without valve's explicit permission and be able to charge a royalty fee for that 'privilege'.
But since they are not, nothing prevents publishers and users from simply going elsewhere with their business, which is why valve have to behave and act somewhat benign if they want to keep their marketshare on pc intact.
This is the whole reason why MS went into the xbox console business in the first place, because they wanted a platform they could control and charge royalty fees on, that's how MS make their money on the xbox.
This is also apple's business model with IOS and how they generate their insane revenue.
Microsoft are after this apple level of revenue, but for that they need to become the platform holder of the PC.
Which they can't do currently because win32 is an open api and you can run unsigned applications on windows through it , MS can't stop you from publishing on pcs with windows installed on them , you don't have to go through them at all.
Enter UWP
UWA is a walled garden api like on on consoles and pc, it has to be signed by MS or it will refuse to run at all
https://msdn.microsoft.com/en-us/windows/uwp/packaging/packaging-uwp-apps , and since it's sandboxed you can't do shit about that.
(and this is then marketing spinned as a 'security' feature, prevent unsigned code from running aka the whole insulting 'add a phishing tool to the game so that it's now capturing my passwords as I type them into chrome' "concern" troll Spencer spouted at build.)
If you want to sell your software , be it on your own store or a third party store, you still have to get your application signed by MS
They hold the keys now.
So if win32 is phased out for UWA (which is what the MS shills always suggest is the future, but it's quite a distopian future) then MS will have successfully taken control of the windows pc platform.
At which point they can treat it just like they treat their xbox, and just like apple treat IOS.
Btw it doesn't mean third party storefronts like steam would go away necessarily (which is the kind of strawman that message steerers love to bring up so they can defeat it to put people at ease about the wrong 'issue'. It just means those will all have to go through MS to be able to operate. i.e platformholder royalties
There is no reason for MS to bring their exclusives to PC when on xbox they collect xbox live paywall fees and platformholder royalty fees on every sale if they can't make that kind of money on PC
Apple have shown companies like MS what kind of money there is to be made if you have full control over a platform with a huge install base (and the windows install base is ginormous, obviously).
It's not in publishers' interests to let this happen. EA did not make origin (to circumvent paying a retail cut) just to end up paying royalties to MS instead.
CDPR did not make gog to do the same. Valve have no interest in having to go through MS (which is why they went as far as making steamOS, which is obviously an uphill battle as they lose win32 compatibility ).
It would absolutely BLOW my mind if valve ever supported UWA, or EA for that matter.
And from a consumer standpoint: turning the open windows platform on pc into a walled garden controlled by MS is obviously a nightmare scenario as well.
So either pc users want to keep their platform open and will reject UWP (as long as UWP implies UWA), or windows will turn into IOS or a glorified xbox dashboard and the people who want to use an open platform will be forced to move to linux (and lose 20+ years of win32 bc in the process)
So be smart, and don't support UWA applications.
And on top of that be smart and support any game made for vulkan instead of directx, because in the scenario where MS manage to push UWA on windows you'll want to have your game library be compatible with linux.