That seems so stupid. The only thing I could think might be the reason is that one could work in some circumstances and get paid 'cash in hand' but if discovered to be working to claim it was voluntary. Say like working as a waitress. No work of any sought would block that possible loop hole.
In Spain I want my wife to learn enough Spanish to help me when I am working as a volunteer. She has applied for Spanish nationality and I would think that working as a volunteer with the police would be looked upon as very favourable when she is questioned as why they should grant the request.
