Here is the thing: e-cigs are chump change to NF, they were not created for us, and their market is not us. It is annoying that they initially embraced e-cigs and then walked it back, but whatever. Maybe this happened when the popcorn lung thing initially blew up, idk.
Regardless, logic says for them (and us too) that diacetyl may be dangerous to inhale, and we did not know this initially. Also that we do not know many of the ingredients in flavorings or whether any others are dangerous for inhalation. Perhaps in a year we will find that peach notes are toxic, or in 2 years that strawberry flavoring causes scabies. NF concluded that rather than struggling and being potentially broadsided by these discoveries it was best to disavow inhalation of flavorings entirely, and thus their lives are made much easier and their liability reduced to nil. In truth I would do the same, were I in that position.
More importantly, NF's market is NOT driven by e-cigs, and their products are NOT designed with inhalation in mind. Why would they post warnings!? Many visitors would not stop to read the full text and then conclude "oh this diacetyl is only bad for inhalation so I, flavoring my coffee, am fine." No, many would look once and say "OH NOES A WARNING WHAT IS DIACETYL IT MUST BE POISON I WILL NOT FLAVOR MY COFFEE WITH POISON!" People do not think rationally about these things - it is the same as those folks who say "PG is an ingredient in antifreeze therefore PG will kill you", or "Vaping is the same as smoking because nicotine".
Jack Daniels does not have warnings against putting whiskey in your eyes, or other cavities; though this is dangerous, and though it does work, it is not what their product was designed for. They know it happens, but it is not their business nor their responsibility. NF has no obligation to post inhalation warnings.