Compounding things is the fact that apparently he did not register them in CA, either. Not sure how this applies, but the report specifically said they were unregistered. It might be that the particular rifles might have been legal to possess, had they been registered with the state (I rather doubt it, but I don't know).
Registration of those things CA calls 'assault weapons' is currently impossible for non-LEOS. The last registration period ended Jan 1, 2001. (Penal Code 30900