There are some great opinions posted so let me add some data from uspto.gov (use 'trademark search' from the menu) and a touch of what I understand of Trademark law after filing a few myself: 53 was granted a US Trademark on the phrase 'Paper by FiftyThree' on December 2013 for trademark categories 21 23 26 36 38 and it appears international category 9 with a first-used-in-commerce claim of March 2013. 53 are obligat…
But the open letter is not. A simple C&D would have protected them; publishing it is purely about marketing.