Cruiselines have the right to sell what REMAINING cabins they have on a cruise to anyone, including a large group, but I don't understand how they can TAKE your cabin AWAY from you if you are a confirmed passenger on that ship and date.
Did you VOLUNTARILY allow yourself to be "bumped" from the cruise?
As Peaches asked, did they compensate you for removing you from this cruise?
When you purchase passage on a cruiseline you and the cruiseline make an agreement, a legal contract. I don't think this can be arbitrarily broken by the cruiseline because they "need" your cabin for a larger customer.