The Seller and I have signed a Letter of Intent to purchase a restaurant.

I then sent my list of due diligence items to the business broker to pass on to the Seller, but now the business broker says that I need to complete a purchase contract before the due diligence has been done. My understanding was that after signing the letter of intent, that due diligence was to be completed. They do say that they can include a statement in the purchase contract to give a period of time for the due diligence to be done. However, what was the Letter of Intent for. Isn't the Letter of Intent a legal document (signed by both Buyer and Seller)? What they want doesn't make sense to me.