The CBP has NO powers of arrest on Canadian soil. They are not armed and do not carry handcuffs. There are local police officers near the CBP desks who do have powers of arrest. They cannot force you onto a plane. If there are any criminal issues you can be arrested by local law enforcement and would be dealt with in the Canadian legal system first. Any US criminal charges or US warrants would have to be dealt with through the normal extradition process. These pre-clearance areas are still Canadian soil. It is also a violation of Canadian law to make a false declaration in a pre-clearance facility. If you are refused entry into the US you will then be inspected by CBSA officers who will decide whether or not you would be admitted to Canada while you arrange a flight home or they could decide to send you back immediately.