Two reasons are float (they get your money before you use the coins) and breakage (they keep lost and/or forgotten coins).
Subscription schemes and prepayments create offsetting liabilities.
That is:
I give Example Co. $100 now.
They book an increase of $100 in their cash at bank, but they also book an offsetting liability -- services owed -- of $100.
However, from a cashflow perspective, $1 now is always better than $1 later.
I am not an accountant, this is not accounting advice.
[1] http://www.digitaltrends.com/mobile/heads-or-tails-amazon-co...
Best Buy, for example, sets that level at about two years. In fiscal 2011, the electronics company recorded $53 million in income from gift-card “breakage,” or cards that are unlikely ever to be redeemed, up from $43 million a year earlier.