The aboriginal land claims situation is in such a mess in BC that it would be foolhardy to agree to the UN declaration without some advance clarification and parameters in place.  Otherwise, the bands would be handed what amounts to an outright veto over most economic development in BC.  No non-aboriginal landowner has that right (viz. expropriation proceedings).

It may be different in other parts of Canada but here in BC there are very few treaties and land claim settlements:  most of the Province is subject to aboriginal claims.  Indeed, there are conflicting, overlapping claims between bands that they have been unable to resolve.