From the information that we have seen, parking lot accidents in California are usually decided on a case by case basis and thus are not always deemed each driver to be 50 percent at fault. If you were in an accident in a parking lot with another driver, it will likely be the two insurance companies involved that investigate and determine fault.
Depending on the criteria the insurance companies use and information collected during their investigation of the incident, they will find fault or how much blame they lay on each driver. California has a "pure" comparative negligence law which means that the fault amount drivers involved in an accident may be assigned is based on their degree of carelessness that contributed to the accident.
If the insurance companies find that each party had some degree of negligence in causing the accident, the responsibility to the other person is normally reduced by the other's degree of negligence. For example, if the other driver was found to be going too fast they might be found 60% responsible for the accident, while if your backup lights were out or not working you might be found 40% at fault.
In a case like this, typically the drivers would have their recovery reduced by the amount they were at fault. So if you are 40% at fault and your damages are $1000, insurance might reimburse for only $600 worth of the damages after taking your portion of fault into account.