Hi there, Andrew, and thank you for your question.
Above all else, the main concern of
Walt Disney World is the safety of the guests and the ability of every guest to have a wonderful experience. The list of items
prohibited at Walt Disney World reflects the goal of keeping guests safe. You'll see items on there that would be a hazard or a nuisance to many Walt Disney World guests.
On the prohibited list includes both a selfie stick and a monopod that does not fit into a backpack for safe keeping. You can bring a retractable monopod or one short enough to fit into a standard backpack. The main difference from my perspective is that a selfie stick projects forward, potentially injuring or interfering with another guest, and in general, can be used by people as they are walking, which can be a risk for the user themselves if they aren't paying attention.
A monopod, on the other hand, is, by its nature, meant to be used in a stable position. Some monopods feature three small feet that are used to rest the equipment on the ground, and those that haven't got the feet are still designed to be used in a resting position, and they point up, not out. They pose much less risk of injuring or interfering with another guest while they are in use.
I hope that clears things up for you!
Anna