I just did some more thinking. Suppose Mr. Popular wins the national popular vote, but Mr. California wins the California vote. The intention of this legislation is that Mr. Popular should win the national election, and thus it would seem that California should want to cast its electors for Mr. Popular regardless of how many or how few states adopt similar policies.
Why, then, cast things in terms of a "pact" which must be signed onto by enough states to swing the election before taking effect?
Changing the rules after the game won't fly. The idea here is that whomever wins the popular vote, gets a majority of the electoral vote, per a pact that states with 270 electoral votes sign on to. For it to work though, Ohio or Florida needs to sign on. Absent that, states with 270 electoral votes won't be there.