That’s a question specifically for Calfee...I think the real issue is that warranties are nice for getting stuff replaced when it fails. Thus, these companies will only warranty things that they have tested and verified will not fail before they extend the promise of replacement (if in their testing it was unlikely to fail). That being said, if you turn it around, just because something is not warranted does not mean it will not work or function as desired. Also, consider that on a tandem, as opposed to a one up, in the realm of braking, you are asking the brake system to haul 130-150kg (maybe more) to a stop, where the brake system is composed of parts that were initially designed and spec’d to bikes where the total system weight was 60-90kg.
Just my hunch, since I am not a lawyer.