The Payment Protocol (BIP 70) describes three messages passed between a payer and payee. The last one is an acknowledgement message sent by the payee.
This discussion talks about the acknowledgement being used as a receipt. In the event of a dispute, the payer can show the payee this receipt to prove payment.
However, unlike the payment request message, the acknowledgement message isn't signed. I may be missing something, but this looks like a weak form of receipt without a signature.
Although the acknowledgement attaches the payment message, the payment message doesn't reference the original payment request, which is the only message with a signature.
It seems like a signed acknowledgement message would provide strong proof of purchase, and would be very valuable from the perspective of consumer protection. Why wasn't this included in BIP 70?