Legally, we're clearly still in a grey area for this.
Ethically... I find it fairly immoral to intentionally prohibit users who have bought your product from using it in ways you don't like. Courts also tend to look poorly on this, but there's a narrow path to walk between enforcing interoperability and allowing innovation.
I guess I'd say the concepts "sale" and "ownership" are at stake here. If I buy something, I expect to be able to extend/modify/repair/dismantle that device in any way I choose. I OWN it.
In the physical world, it's nearly impossible to put some sort of non-electronic lock on a device that would prevent the user from doing just that. There's no reasonable way to make a cup that could only hold Sprite.
So that status quo up until the last 70 years or so has been "buy means buy" and you own the thing.
Now we have products with incredibly complicated digital locks, though.
It turns out you can prevent users from putting something other than sprite in their cups with a complicated enough digital enforcement mechanism. The question is "should you be allowed to?".
And that's actually a hard question - The answer depends a lot on how many people need to interoperate with you and your products (your marketshare and possible monopoly status), as well as whether you have a genuine benefit you're providing your consumer by limiting that cup to just Sprite.
So legally speaking, Keurig just settled a case about this exact issue (they're out 31 million, btw). This is not nearly as "problem free" as you so clearly want it to be.