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.
They also forced developers to never price lower on any other platform than steam as a condition fire selling on steam. Which is not only unethical, it’s illegal. Also the secret hardware changes to steam deck which people usually try to justify, but was shady no matter what.
That’s not entirely true. Valve forces devs to not sell Steam keys lower on other sites without also going on sale on Steam in a reasonable close amount of time.
I know it sounds the same at first, but it’s a drastic difference. You can generate as many Steam keys as you like and sell then on other sites, Valve won’t see a single cent from these sales. They however still provide their online services and servers for free for all those keys sold on other sites. It is quite reasonable that they force you to match prices since they literally are losing money (albeit not much) if you sell on other platforms. And I don’t mean lost sales, but infrastructure cost.
And additionally is this rule pretty much never enforced. AAA studios have special deals and indi devs aren’t worth the hassle.