They did this to provide message-level integrity. OAuth 2 switched to Transport-level confidentiality/Integrity.<p>It's worth noting that message-level integrity was not a design goal of OAuth 1; it is was a consequence of being based on OpenID 1/2, which were explicitly meant to run on HTTP without TLS so that they could be adopted by blogs. This was pre SNI, and pre cheap certs, so requiring HTTPS increased the hosting cost of a blog by an order of magnitude.<p>When the constraints changed such that requiring HTTPS was feasible, it greatly simplified OAuth. Some of these simplified proposals for OAuth became the input for OAuth 2 (where complexity was subsequently added back in the form of variants to support new use cases).<p>Relying on message level integrity in a protocol where such a thing was basically a side-effect of avoiding hosting costs would make me <i>very</i> nervous.<p>The clearest issue I can point to is that there is no response message integrity in MasterCard's system - an intermediary can block requests to MasterCard and give back fraudulent responses (yes, of course that payment went through!). This throws a ton of application-dependent security considerations into the system.