It's a credo of the Fairy Faith.
If ever you should happen into That Land, Don't eat the food.
To eat it would be to bind yourself irrevocably to that world, from which you can “never return to your ain countree.”
All the stories agree that the Tribe of Witches are exempt from this taboo.
We have, shall we say, a special relationship with the Secret Commonwealth. As people of the betwixt-and-between, it is given to us to pass from world to world with something (dare I say it) akin to impunity.
Scottish witch Isobel Gowdie said of her visit to Elfhame: There I got meat, more than I could eat, nor did this hinder her comings and goings in the least.
Old Craft would have it that this right of free passage derives from being ourselves of That Blood, half-elven, from whence we draw our Otherness.