This is reportedly a side-effect of FDA guidelines and how drug studies are conducted. FDA guidelines asked drug companies to list "all possible side effects", which meant that if a single study participant had a bad meal while taking the experimental drug, and suffered diarrhea as a result of the bad meal, then diarrhea became a possible side effect of taking the drug.

The FDA adjusted their guidelines in 2015 in response to this (https://www.chicagotribune.com/business/ct-confusing-drug-ad...).