I think it was an interesting letter, but I can fully see why AMHA would not publish it and AMHR would.
It is a letter that summarizes some very scientific material. The information is complex, and in such a short letter, the reader may not be able to fully understand it (at least I didn't). And, if I was AMHA, I would not want AMHA horses to be compared to AMHR horses in a what appears to be a negative light, when there is no way, from the information presented, to see if it was a 'fair' comparison.
It states, "The prevalence of these 4 ACAN mutations among AMHA miniature horses may be as high as 40% or more."
For the AMHR horses it say, "Among 44 random horses from the AMHR registry (with unknown dwarf status) two tested positive as carriers fo D2 (N/D2), but none were found that had any other ACAN mutations in any combinations."
From only what is presented in this letter, it is not possible to see if this is comparing 'apples to apples', between the organizations' horses, or if it is 'apples to oranges'. There is no mention in the AMHA part about the number of animals in the study, whether the AMHA horses were of unknown dwarf status, or even if they were comparing the exact same 'issues'.
If I was AMHA, I would not want to print a letter that painted the animals in my Association as 'inferior' to any other organization, not based solely on a non-scientific person's brief presentation of someone else's scientific work.
I would hope though that AMHA does run a full article on the dwarf study, and that they cover the issue fully, the good, the bad, and the ugly. But again, I do fully understand why they would not want to print 'the letter',