The treaties with the crown require the crown consult with them before adjusting them. This means that Albertan secession can't happen without their consent, as it would by definition, completely and unilaterally adjust the terms of those treaties.

The treaties were made in perpetuity, and if you are going to not hold up the crown's end of the promises, the FN's side - giving the crown and Alberta governance over the land - needs to be reverted as well.

Contracts require both sides to adhere to them.