Why do I have to pay water purification charge if I let rooms?

You have to pay water purification charge for business premises if you let rooms

This is often the situation for student accommodations. In that case, the occupants do not have to pay water purification charges themselves. However, you are entitled to recover the water purification charge from the person to whom you let the room. This is stipulated in Article 122d(3)(b) of the 'Waterschapswet' (Dutch Water Board Act).
The occupant does generally pay water purification charges if the room he or she uses has all the necessary amenities.
By amenities, we mean at least a bathroom, toilet and cooking facilities.