Most things in Citadels can be profited from: Markets, Clone Bays, Reprocessing, Corp Offices. However compression is always free, regardless of any other taxes.
It would seemingly be simple to implement a compression tax, just like reprocessing by working out the market value of the goods and setting a rate based on that?
This is just a basic thought on how it could be done. It seems to simple to work though if no one else has suggested it. Maybe someone else has a better idea on how to go about this?
I myself have used and abused citadels to compress ore/ice and I know plenty of others who have also owned up to doing it and reprocessing elsewhere. I feel the citadel owners are losing out on this one.