Capacity controls

Question:
Aeroplan told me there are no seats available but I can buy a ticket on the flight, why is that?

Answer:
Well, my dear, it is called capacity controls.

Aeroplan Terms & condition state:

16. Capacity controls also apply. This means there are a limited number of ClassicFlight reward seats available on a given flight. There may be instances when flights are not completely booked but all available reward seats have been claimed. Capacity controls do not apply for ClassicPlus flight rewards.

In other words there are a limited number of reward seats on any flight.

I prefer the reward cards that allow me to book anywhere i.e. priceline.com, expedia, travelocity, travel agency etc. and redeem points for travel. Forget about Aeroplan too restrictive.