I think it is pretty unreasonable. As you note, there is no technical reason to deny the Bernie campaign access to their data. The Bernie campaign has fully indicated they want and are willing to cooperate with a third party investigation into the data breach, which would require investigating both campaigns, the DNC, and NPG VAN. Given they are already willing to share everything they know about the incident, there is absolute no legitimate reason for the DNC to intentionally sabotage the campaign.