If ISP's were forced to actually be responsible for IP ranges and to actually clean (like some services, spamcop, etc. forces them to) then the very definition of the fast flux botnet (of an ever-changing network of compromised hosts) would become mute :)
Eighteen years of handling abuse at large providers forces me to conclude that while what you say is technically correct, it is utopian. Beyond that, who does all the forcing? I seem to recall a lot of “we are not the internet police” out here not too long back so it isn’t the RIRs. Clearly, from this thread, it isn’t the registrars. Blocklists like spamhaus tend to get extremely bad press when they call out a registry for poor abuse controls, so it isn’t the registry either I guess. [And I know a lot of registries, registrars, and ISPs who do a great job keeping their particular corner of the internet clean] —srs