That's not crazy at all, just a standard warning... It's just Apple washing its hands of any blame should you incur charges on your phone bill while downloading your iOS update via your cellular network.
It's a bit odd (but it doesn't mean anything) that in this instance, they wrote "will" apply—most warnings of this type use the much less ominous "may" apply.
...Or if iOS automatically communicated with your wireless carrier and adjusted itself accordingly.
Heck, if they got visual voicemail to work properly, and years ago at that, then I'm sure it would be quite possible to issue a simple true/false query to the network and make the result visible to iOS so that it could behave accordingly.
However, I'm inclined to believe that Apple will always err on the side of caution and warn you about potential data charges, even if the the technology to avoid the warning (when unnecessary) is available. This helps Apple avoid a scenario where hundreds or thousands of pissed off customers who get charged a fee for exceeding a data cap cry foul and attempt to band together and start a class action against it, against the carrier, or against both.
I have the Overcast app in addition to the native app. Each has the warning regarding potential data charges if one enables downloads over cellular.
In case the OP cited, I was tempted to reply that it was merely a standard/canned warning that Apple probably requires for all apps ... aka "cover your ...".