Not saying I agree it's legal, or right, or equitable or lacking basic unconscionably, but...
I think they are very nice to offer free cancellation of membership without cost. I think Aeroplan Rocks!
https://www3.aeroplan.com/terms_and_conditions.do
Aeroplan assumes no liability to members whatsoever by reason of the termination of, or amendment to, the Aeroplan Program, in whole or in part, the addition or deletion of reward partners (including Air Canada), limitations on the availability of flights or seats, changes made by Aeroplan Partners to their terms and conditions, or any change made in accordance with sections 6 to 8 below.
Aeroplan specifically reserves the right to unilaterally amend the provisions of these General Terms and Conditions regarding:
the nature of the Aeroplan Program
partner, supplier, or vendor obligations and liability
rights and liabilities of Aeroplan or members
communications
enrolment and membership conditions, restrictions and sanctions
the accumulation, redemption, expiry or cancellation of miles
characteristics or attributes of miles
rights to or associated with miles, errors or delays with respect to rewards or the crediting or debiting of miles
purchase of miles
the termination of the Aeroplan Program or membership, or the subjects under these General Terms and Conditions that may be amended and the manner of doing so
governing law and jurisdiction.
Aeroplan will give 30 days' notice of any changes to these General Terms and Conditions, and a comparison illustrating the changes.
If a member does not accept any change, the member may cancel membership in the Aeroplan Program within 30 days after the effective date of the change, by way of notice to Aeroplan, without cost, penalty or cancellation indemnity.