Because the annotations angle is a red herring? In the Oyez transcript of oral argument[1], Citron (for the respondent) made what I believe to be a relevant remark disputing the petitioners' assertion that legal annotations would be more expensive:

> One is the actual useful versions of these codes are already plenty expensive. There's a lot of discussion of the cost for a printed volume, but online access, which is what really most practitioners need to use, most people want to use, it's much more expensive than the $400. But, even accepting that the price is lower, I think that favors us, because what's going on there is an exchange of -- you're going to accept a price cap in exchange for the right to publish this officially, not for publishing the annotations, because Westlaw makes the annotations and is allowed to charge six times as much. Lexis isn't going to agree to do the annotation work in exchange for a price cap. What it wants for the price cap is the right to publish it officially.

As I understand the game being played prior to this decision (to be sure, IANAL): although annotations undoubtedly have market value, the market (for liability and other reasons) will nevertheless tend towards official sources, and if you're the official publisher granted monopoly, who cares if the thing being sold has a price cap ($?) when the subscription to the service that provides access to said official thing won't ($$!)...and even if said subscription cost was pragmatically constrained by market forces, the dragnet scales from typical legal/government/academic players in the arena to capturing a proverbial crap ton of otherwise unmarketable endusers on the receiving end of the law ($$$!) while taking share away from direct competitors ($$$$!!). Then make a free, unofficial version available in an attempt to pacify public dissent, but seed a mandatory disclaimer that it may contain errors, so anyone with skin in the game who can pony up will be compelled to do so.

[1] https://www.oyez.org/cases/2019/18-1150