I do think that to answer this question, you would have to contact the NCHFP directly.  All of their recipes are research based, so it could come down to a safe acidity level, or it could be another reason.  
There is a big degree of difference in the acidity of apples, and I know there has been some attention given to this lately - but this doesn't answer your question, and I'm not sure I can give you an answer.  

Ball recipes are also research based, and safe - so you could use their apple butter recipe if you'd like, where there is a smaller amount of acid solution used: 
Apple Butter Recipe | Ball Mason Jars

I do think that it is important to follow a tested recipe, that is a big takeaway.  Thank you,