Yes, the Air Miles rewards program is absolutely free. There are no hidden costs required to join the program. Many Air Miles credit cards also come without annual fees, making them free to use as well.
However, it is best that you pay attention to your particular credit card’s terms and services because some premium credit cards such as the BMO Air Miles World Elite Mastercard or the American Express Air Miles platinum do come with annual fees, though these may be waived for the first year.