[thinkharderdev]

Right, they let you do it now but iirc that is a relatively recent feature that was resisted for a long time. And for good reason I think. The purpose of certificate validation is to verify that the remote machine is who they say they are. But those guarantees are already provided by the VPC protocol. In order to impersonate a target instance you would need to MITM the traffic, which isn't possible in a VPC.