Western Davey has put his finger on something that differentiates good sappy from bad sappy. Emotional loading. When you take an inherently beautiful tune and slip blind-eyed thoughtless provincial chauvinism into it, ye get a messy combination for sure -- "America the Beautiful" is a prime example, although the loading isn't that bad -- crowning good with brotherhood is actually a fine sentiment, despite the chauvinism. Another is the Marseilleaise, a very fine tune, with some stirring lines, which then slips in very deftly a few well turned hate-and-kill-and-fear lyrics to tie you up in the best nationalist knots.

A lot of beautiful religous music has a similar impact -- you get into the musical part and take "marching as to war" on board as an uninspected and unsalubrious emotional cargo!

These aren't actually 'sappy' so this is thread creep, but 'bad sappy' surely involves a similar mechanism with different emotions, like cloying sympathy and getting the listener to resonate with profound stupidity.