They could have easily crammed the Steam Deck full of stuff to make it hard to use for piracy - locking down everything, making it usable only to play games you legitimately own, force you to go through who knows what hoops in order to play games on it. That’s what Nintendo or Apple or most other companies do.
But they didn’t, because they realized they didn’t have to. It’s 100% possible to put pirated games on the Steam Deck - in fact, it’s as easy as it could reasonably be. You copy it over, you wire it up to Steam, if it’s a non-Linux game you set it up with Proton or whatever else you want to use to run it, bam. You can now run it in Steam just as easily as a normal Steam game (usually.) If you want something similar to cloud saves you can even set up SyncThing for that.
But all of that is a lot of work, and after all that you still don’t have automatic updates, and some games won’t run this way for one reason or another even though they’ll run if you own them (usually, I assume, because of Steam Deck specific tweaks or install stuff that are only used when you’re running them on the Deck via the normal method.) Some of this you can work around but it’s even more hoops.
Whereas if you own a game it’s just push a button and play. They made legitimately owning a game more convenient than piracy, and they did it without relying on DRM or anything that restricts or annoys legitimate users at all - even if a game has a DRM-free GOG version, owning it on Steam will still make it easier to play on the Steam Deck.
Valve is one of those companies that I genuinely believe makes a strong argument for ethical capitalism being possible. Sure, they have some shitty things, but overall they do treat developers and customers reasonably well, they provide hardware and software that is easy to use and non-abusive (not filled with spyware and data harvesters, doesn’t use advertising, is well maintained, etc.). If we could obliterate all of the other major conglomerates and replace them with people/companies that understand that you don’t have to be a massive pile of shit to make money the world would be better off.
Valve argued in court that you do not own any title in your library and that they are a subscription based service. That’s not very ethical.
Valve is not publicly owned, I don’t think you can equate commerce to Capitalism.
Commerce conducted in a capitalist economy is inherently capitalist. Being publicly traded is not strictly required, though it might be the most common form of corporate structure under capitalism. Individuals, partnerships, privately-held and publicly-traded companies can all own capital. Valve’s assets are not owned by a government, its business decisions are made privately and it operates in a free market. Those three factors are pretty much the definition of capitalism.