They have a brand image to uphold. If anybody were to be able to open up a franchise and run it badly (ie, have rats running around), this would have a serious

They have a brand image to uphold. If anybody were to be able to open up a franchise and run it badly (ie, have rats running around), this would have a serious negative image on the whole brand and would sway people away from eating at the same branded joints again, even though other joints are run by different people. As such, it is necessary to vet the potential franchisee to ensure they're going to support and improve your brand image rather than damage it. Requiring a "deposit" as such is part of this, as the franchisee will need to at least recover the amount invested by providing good service.