Imagine buying the SM for a teenager in your family that doesn't have a Steam Account yet.

First: Imagine a situation that won’t happen. A teenager without a steam account?

Second: They could easily allow any account to log in as long as the account that bought it is an actively logged in profile.

If you think that 'teenager' is too old (which I am skeptical about, considering the current popularity of Roblox), replace by 'kid'.

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More complications, more points of failure.