Why do people say you shouldn't reheat certain meats?

I've never heard that. If it's been properly cooked & cooled, reheating is fine. The thing you need to watch out for is refreezing. If food thaws you can't refreeze without cooking it first & then only if it has not yet reached room temp. Outbreaks of ecoli & samonella have been linked back to shipments of meat where the trucks freezer broken down but they just refroze the meat & shipped it off to places like Jack-n-theBoxes & Wal-Marts all over the country.