The canister is actually pressurized to balance the pressure inside the container with the pressure inside the tennis balls. This will prevent the balls from going flat. The pressure inside the balls is 12 psi over the normal air pressure (for comparison, the pressure in a car tire might be something like 32 psi). The rubber shell of a tennis ball isn't enough by itself to keep the ball bouncing high, so the pressurized air inside the ball makes it springier. Even though the rubber shell may seem impermeable to air, it isn't. It slowly lets out air until the pressure inside the ball equals the air pressure around it. The pressurized can prevents this from happening until it's opened.