That's capital. It gets given back to investors, who invest it elsewhere. They don't just stick it under the mattress. It is moved from Apple to a place where it can be put to use. With a buyback (or dividend) apple is saying "we ran out of productive ways to spend this money"
Even if they did just keep a few billion dollars of physical cash in a vault somewhere, that would just increase the purchasing power of the remainder. The total value of all currency in circulation (all forms combined) tends toward the total value of all of the goods available for purchase.
Taking money out of circulation is a bit like making a loan to everyone in proportion to the amount of currency they hold; when you do decide to put it back into circulation, that increase in value (deflation) represents the interest you receive on the loan.
