CBP has no right to arrest anyone at Canadian pre-clearance ports. If they don't want to admit you to the US, their options are:
1. deny you entry, and send you "back" to Canada, in which case CBSA will decide whether to admit you to their country,
2. ask Canadian law enforcement to arrest you, which can only be done if you have violated a Canadian law,
3. let you board the flight and then arrange for CBP to arrest you at the destination US airport, which is very rare and usually only for cases where you have an active warrant for a serious crime.