In September I'll be riding the PCR from San Francisco, south to the border, and have found that many of the campgrounds marked on the Adventure Cycling maps don't have bike/hike sites (I just presumed if they were marked they would).
As I recall, the maps specifically say hiker/biker where that is the case. Do you mean that ones that are marked hiker/biker actually are not? In that case the AC addenda should say so. If concerned, calling ahead each day would be an option.
When I did the PCH I was usually camping with other cyclists and someone else usually called ahead to verify what the situation was. I never needed to bother, but calling ahead each day wouldn't be to big of a deal. That said, when alone I typically don't bother. I start early enough each day that I get to camp early, so if I find a place full I have time to press on to the next one.