Except that the analogy doesn't really fly for desktop-pc's (or laptops for that matter): There are typically multiple customers using one desktop / device. If you share a computer with your whole family, everyone could play the game on their own account, even without having knowledge of others also playing the game. It's different for smartphones since those are more often used by a single individual and are typically not shared.How is that and "edge case"? GI's Terms and Conditions of use are very clear "you agree not to use the Services in order to.... create more than one account per device". That seems crystal clear to me.
If it is against GI's T&C's it isn't legit.
If the clause is rational is an entirely different question. As is GI's right to decide if they wish to enforce it now or in the future, or simply have it there to potentially use when other violations are found.
To use Ivan's grocery store analogy, Walmart may have 1,000 packs of special offer corn flakes on the shelf, but if a term of the offer is "limited to three per customer", you can only legitimately buy three. Windows Store or Google Play etc. may allow you to install multiple copies of the game, but its in breach of the T&C's if its on one device.