Subway made us police custom uploaded background images on gift cards. This was a decade ago. They don't give you any rules to go by, just a sense that "if something isn't kosher don't let a private user upload it to use as the background image to a subway gift card that they are purchasing with their own money for their own usage". Brand "rules" are the reason why we will replace any profanity you put in the delivery message. Brand "rules" are why you can't buy a Subway (or Chipotle) gift card with "Bitch I know guac is extra" for example.
This is absolutely "brand safety" stuff.