What is the reason that Intel would allow speculative instructions to bypass the supervisor bit and access arbitrary memory? That seems the root cause for Meltdown.

Is it that the current privilege level could be different between what it is now, and what it will be when the speculative instruction retires? If so then that seems a thin justification. CPL should not change often so it doesn't seem worth it to allow speculative execution for instructions where a higher CPL is required.