According to the text unearned revenue is a liability because although the customer purchased the item technically the item has not been claimed. There are many things that can go wrong and the company may end up not gaining the revenue. Unearned revenue is classified as a liability because it is entered as a debit to the cash account and a credit to the unearned revenue account. When companies do not deal with unearned revenue in this type of way, and do not recognize it all at once, the revenue and profit would become overstated, and then understated for the additional periods in which the revenue and profit were suppose to be recognized. This also causes conflict in the matching principle, revenues are being recognized all at once, while expenses are not being recognized until the later periods.
Unearned revenue is initially placed on the balance sheet as a liability, once the product or services are complete then the company will place the charges on the balance sheet as earned