I banged my head on this one for a bit as well. Why let a person who is already voting to keep Davis, a chance to pick his replacement as well? but, this is the way the law works. Got to trust Hiram on this one. ;-) A lot of No voters won't bother to select a replacement candidate anyway.
That's one of the reasons the Democrats and the unions are so terrified of a special election. They know that anti-Davis voters (especially Republicans and Independents, but also minor party members and some Democrats) will be highly motivated to get to the polls. Whereas the majority of Democrats will be depressed and uninterested in trying to rescue Davis, so their turnout will be quite low. Add to that an expected dropoff of 8% to 10% from the recall question to the replacement question, and even a prominent Democratic candidate would have a tough time beating whomever emerges as the leading Republican.
This special election neutralizes the Democrats' normal edge in voter registration in California. Only Feinstein would be an odds-on favorite to retain the governorship for Democrats.