Most franchises do it (including the Celtics)
Not exactly.
The Celtics season ticket package comes with two preseason game tickets and 41 tickets for regular home games.
Here's where the interest-free loan comes in.
Shortly after the team locks in a playoff spot (but not before), they send out a full set of playoff tickets for all four rounds, and they bill as if every game in every round will be played. If each round goes less than seven, and/or the team is eliminated, the organization DOES hold on to the money for the unused tickets and applies it to the next year's bill, or they will issue a refund on request.