'Ringfence'
Problem is, by 'ringfencing' the Ring doorbell, you're left with a £200 paperweight which isn't able to fulfill all the promised magical things it was intended for.
Amazon have you over a barrel here, even if they forced everyone tomorrow to re-sign the EULA/T's and C's to explicitly state they reserve the right to share your footage with whomever their heart desires, I can almost guarantee at least 50% of their customer base would probably (begrudgingly) smash the 'Accept' button so as not to turn their expensive doorbell into a stupid one. As far as I know, there's no way to hack these things to use a third-party service, thus Amazon's cloud-based wizardry is a complete necessity for it to actually function.