1) The proposal is not treating them the same as legally held guns, it is far more restrictive. It is banning the sale of them. So a new user or someone who gets a licence could not buy or even replace an existing crossbow.
2 ) The licencing system has not been set out. What are the costs and how will it work. It's likely to put off ownership so people give up. Is stamping down on outdoor pursuits sensible?
3) The legislation will take time, a licencing scheme will be costly and if it is policed this will take up more resources.
4) Relating the point 3, would the cost involved be better spent on reducing crime and criminal behaviour? Will someone not commit a crime because they can't use a legal cross bow or will they use a knife, a different type of bow, or something illegal?
5) Will it set a precedent? Would some knives be next such as daggers or a collectable knife need a licence - after all no one needs them and they could harm someone...