It's very "pirate of Silicon Valley", i.e. very old-school Apple: "so sue me". Regardless, the right answer is to engage privately first, to see why they missed that 5% and if there is any way to find agreement, not to go pedal-to-the-metal with public shaming.
As someone who's tried to contact Apple before, posting publicly is about the only thing that would ever work. Good luck getting in contact with anyone there who'll speak to you. They're worse than most other companies, and most other companies are already quite bad.
For an individual, yeah. Apple belongs to a number of standards bodies and consortia, though; when they deviate from an existing standard, they usually do do more deliberately than this letter implies.
I just don't get why you're so stuck on this one point on etiquette. To me, saying "We tried to contact you to no avail" is ruder than omitting the line.
