Hello,
It is not due if the guests are staying for free, BUT if you also rent your home, some hosts prefers to ask for it anyway, to avoid incurring penalties (because it’s difficult to demonstrate that those guests are friends / home exchangers, and not under-the-table clients, i.e. tax evasion). I agree that asking for a receipt is a good way to avoid any abuse.
PS. If you want to discuss the matter with the host, you can still refer to this link, which summarizes everything: Imposta di soggiorno per chi fa scambio casa in Italia: è dovuta? 🤔