You're not buying "a year's worth." Adobe can't roll a truck up with all your future project rendering hours on it and dump them on your lawn, such that they would have a valid legal argument of "you can't cancel, we already gave you the whole thing." What Adobe are giving you, each month — each second, even — is the DRM licensing functionality built into Photoshop continuing to spit out a "valid" signal. Because that activation is a continuous online process, you
receive that service on a second-by-second basis (or maybe at most on an online-activation-check-granularity basis.)
That being said, maybe we're talking past one-another here.
Where I come from (Canada), even if you prepay for a service that charges annually (no "annual charged monthly" language needed), as long as that service can be common-sense-construed as delivering value on a finer granularity (by the month, by the second, etc), then if you only use that service for some fraction of the plan length, and then cancel it — you are then legally obligated to a pro-rated refund of the remaining plan length. So if you cancel an annual-billed service after a month? You get 11/12ths of your payment back. If you subscribe to a monthly-billed service on January 1 and cancel on January 2? You get 30/31ths of your payment back. Etc.
Under such a legal doctrine, there is no difference in the total amount owed between "billed monthly" when subscribed for one month, vs "billed annually" when subscribed for one month and then cancelled, vs "annual, billed monthly" when subscribed for one month and then cancelled.
If you're curious about the set of countries where this doctrine applies, here's a page from the Microsoft Store support outlining the set of countries where they will give out pro-rated refunds for subscriptions: https://support.microsoft.com/en-us/account-billing/countrie...
(And if it isn't sickening to you that in general, corporations will write logic into their billing systems to support this, and then only activate that logic for countries where they're legally obligated to do so, while — now with intentionality — continuing to squeeze everyone else for services they've knowingly already cut off... then I don't know what to tell you.)
---
And yes, if you're wondering, there are a few exceptions to this pro-rated refund doctrine.
One is real-estate leasing — because chancery courts are weird and make their own rules; but also because a lot of the "work" of being a landlord is up-front/annual. (Though, admittedly, we also have laws here that force real-estate annual leasing contracts to revert to month-to-month after a low set number of years — usually 1 or 2 — with the month-to-month lease rate carried over from the "annual, paid monthly" rate.)
The other is for commercial leasing of assets like vehicles, construction equipment, servers, etc. This is because corporations have much more predictable optionality, sure — but it's also because corporations don't "deserve" protections in the same way individuals do. (Same reason investment banks don't get the protections of savings banks.)