It's not just a scare tactic, it's how these companies operate. They will illegally refuse to service products they have an obligation to. Until there are massive class action lawsuits, this isn't changing.

Even if it's illegal it doesn't override user protection laws, so the statement has no effect and they'd need to prove your SDK usage caused an issue with the product to legally deny warranty.

The contract is under Japanese jurisdiction. Does anyone here know Japanese law in regards to something like this?

No, I don’t think that’s how that works. Your contracts with your customers is under the jurisdiction of where your customers live, not where your company is headquartered. It would be way too easy to side step any law they wished that way.

Plus, must camera manufacturers sell through an american subsidiary. I know thats how Canon works.