From the point of view of financial reporting (i.e. accounting income that you see on the financial statements), holding the liability indefinitely is not a good thing as it negatively affects the leverage ratios and implies a future obligation. The new accounting requirement that allows companies to estimate the so called 'breakage' (i.e. anticipated unused balanced) and recognize that as income over time rather than keep it as an indefinitely liability.
Legally, there's also the concept of escheatment, which depends on the country/state that tells a company what to do with the unused balance. In some cases, the government may claim this balance.
To your question about paying income tax on income, most tax codes would require the companies to pay tax on the amounts when they receive the cash rather than when they satisfy the obligations. So it's likely that they've already paid tax on this amount.
