This is their official response.
Common Problems
* Claimed this season’s pass when I didn’t mean to.
- Accidentally clicked Activate while browsing the pass.
- Activated the pass but haven’t claimed anything; can this be reversed?
Once claimed, a Diablo IV Battle Pass cannot be revoked. Customer Service is unable to assist with removing or re-issuing already-claimed passes.
What they fail to realize is this is just like what EPIC got sued for, and lost 250M USD. The crux of the problem is that Click Through agreements or Click Wrap agreements need a confirmation that you are agreeing to purchase or otherwise obtain something by clicking on it. Due to the fact battlepass has a monetary value, and you are obtaining something, it must have an agreement as such at time of purchase. There is a reason you have to agree to when you do not have a battlepass waiting to be used to purchase the item. There is always terms and conditions that come with that purchase.
What we have here is Blizzard dropping the ball on adding the agreement of purchase due to shotty programming (who is actually surprised?). How do we know this? If you do not have the battle pass from the ultimate edition, it will prompt you for the click through to purchase. This means that they just failed to test their code and ensure that there is a click through in place. They also have a click through when you buy ANYTHING from the store, even if you have the free coins that came with the Ultimate edition.
Blizzard is trying to pass the fault on to you consumer, which should come as no surprise to anyone. This is a colossal problem for them, and they are trying to divert the blame by not assuming the liability of the problem. This is exactly what caused EPIC to get sued. The problem was "Epic used a host of digital design tricks – dark patterns – to charge consumers for virtual merchandise without their express informed consent. ". Due to the fact we did not give our expressed consent to consume the battlepass voucher, we are falling under the same precedent as the EPIC games lawsuit.
What I suggest you do is open a ticket, and write Blizzard and let them know you will be signing on a Class Action if one arises. Considering there is prior precedent for these actions, it should be a semi easy slam dunk for any law firm who chooses to take up that mantle. I also suggest you put in writing that you are asking for it to be refunded, so they can track the case load. You can file a ticket here: Blizzard Support - Contact Support
For further reference about the EPIC Games lawsuit:
https://www.ftc.gov/business-guidance/blog/2022/12/245-million-ftc-settlement-alleges-fortnite-owner-epic-games-used-digital-dark-patterns-charge
https://www.fastcompany.com/90866732/epic-games-fortnite-design-lawsuit-settlement-money-claim-how
EDIT:
1.1.0c Build #43556 (All Platforms) - July 26, 2023
Updated the activation of the Premium Battle Pass with a confirmation pop-up.
Confirmed it was a bug. They better refund our damn battlepass.