Some processors legitimately restrict access to HACCP information, and may do so during audits too, where they believe that parts of the process are confidential intellectual property.
As Zanorias noted, it may be possible to cover this point via the processor's certification if it is an appropriate standard - certainly all of the GSFI-benchmarked schemes would arguably sufficient independent verification of the HACCP system. Commercial pressure can sometimes help, but only if you're purchasing power is very significant 
You could try asking them if they can provide simplified details - some sites with complex and/or confidential processes will issue a simplified process flow etc that shows an outline of the process without giving any specific details away.