Ok so if they are now only charging for the first install, why aren't they just charging an extra fee per sale? Wouldn't that accomplish effectively the same thing? (And actually work out in unity favour since not everyone who buys a game downloads it)
Because they realize that a huge number of their customers are small indies, and they want to be able to squeeze them - the majority of their customer base - not just the minority of big companies (who are also the most likely to fight back legally).
Just look at how their scheme squeezes smaller, poorer developers way more than big ones. If Unity went by points like, say Epic does with Unreal, they could shake down the big developers… but wouldn’t get much out of the indies.
Which is the opposite of what smart companies like Adobe do. You facilitate the small players in hope that they grow big and keep using your products at a larger scale.
That's probably pretty negligible numbers. In fact I'd suspect that the number of people who buy a single copy that they then install on multiple devices is lower than the number of people who buy a game and never play it.
It's also much simpler to implement and the numbers are verifiable. Unless... that's exactly what Unity wants; just "trust me bro this is the correct number" kind of deal.
Also Steam Deck - every install and uninstall is considered a new computer. That's true for Linux gaming using Proton in general, but the rest of Linux gaming is not as relevant.
The only major reason I can think of is people playing on PC and Steam Deck, using the cloud save to play on both. Sometimes I want to play the same game on the big screen and sometimes in bed.