Very often, a Franchisor requires a significant portion of the product a Franchisee to be purchased either from the Franchisor directly or from an...

Very often, a Franchisor requires a significant portion of the product a Franchisee to be purchased either from the Franchisor directly or from an "approved" supplier. Also often, the Franchisor makes additional income as a result of these requirements. This requirement has resulted historically in a significant number of lawsuits (Franchisee's suing Franchisors). Why would this be a legal concern, and on what basis would the Franchisee have standing to sue the Franchisor?