I don't think it was immediately purged, but we do have the facility to do that due to some labour law or other. But to be honest I have forgotten how to do it. What I do know though is how to secure it so that it can't be over written.
This incident though only came to light 2 days afterwards and was not a reportable event (which has now changed in the US it would have been in europe) so by the time they started pulling the data in the CVR was over written by the previous 2 days flights. So it wasn't a crew commanded deletion just a normal usage cycle The FDR records 50 hours worth of data on a cycle so it was still available.
Personally I wouldn't do a visual onto a multiple runway airport its just way to easy to screw it up in the day time never mind night. But the way they deal with approaches and landing clearance in the USA is quiet different to what I am used to.
SFO has a colossal history of issues, the locals seem to take an almost perverse joy out of it. Others hate operating in there. I have seen a US pilot write that when her alarm goes off in the morning to fly into there she already knows that 30 odd holes are lined up in the accident swiss cheese model and she hadn't even had a shower yet never mind start an engine. Airport layout, ATC attitude, procedures, shite weather... the list is colossal. Nothing changes though and the excuse is that they are known problems and the pilots know about them and should mitigate them.... And if its left to the pilots eventually someone will screw things up. Throw an Air China into the mix and chaos ensues. Which might be amusing listening to in the comfort of your computer room it certainly isn't when your in the middle of it.