I wouldn't say Ziegler move is a very bad. How i understand our leftbacks line is totally completed. Ziegler, De Ceglie will compete for the starting place and Traore or Grosso leave.
The definition of a Bosman is a free transfer when a player's contract has expired. Cassano's contract didn't expire in January, they never do. If Sampdoria let him leave for no fee that's their choice, but they and Milan, as the buying club, still ended up paying €5m between them to settle the clause with Real Madrid specifically because he was not a Bosman free transfer.