It turns out that there are a few terms and factors involved that help to determine whether account credits provided by a business are taxable.
To begin, there are two types of virtual currencies: convertible (open) and non-convertible (closed). The key difference between these two is whether or not they can be directly and legally converted to fiat. This is made especially clear when you see disclaimers like, "Not redeemable for cash." on things like vouchers. Since the currency provider has restricted the ability to exchange for fiat, these would be considered a closed virtual currency.
Closed virtual currencies have far more restrictions than open virtual currencies do. Their use tends to be exclusively within the business (through receiving goods or services); the business controls the creation and distribution; the business can choose to stop supporting them; and the business determines how they are valued within the business. The emphasis is essential as closed virtual currencies are not taxable and are essentially valueless to the IRS.
How to determine what type of virtual currency you have is also made clear in Internal Revenue Bulletin: 2014-16 under section 2 of "IRS Virtual Currency Guidance":
IRS Virtual Currency Guidance
The Internal Revenue Service (IRS) is aware that “virtual currency” may be used to pay for goods or services, or held for investment. Virtual currency is a digital representation of value that functions as a medium of exchange, a unit of account, and/or a store of value. In some environments, it operates like “real” currency — i.e., the coin and paper money of the United States or of any other country that is designated as legal tender, circulates, and is customarily used and accepted as a medium of exchange in the country of issuance—but it does not have legal tender status in any jurisdiction.
Virtual currency that has an equivalent value in real currency, or that acts as a substitute for real currency, is referred to as “convertible” virtual currency. Bitcoin is one example of a convertible virtual currency. Bitcoin can be digitally traded between users and can be purchased for, or exchanged into, U.S. dollars, Euros, and other real or virtual currencies. For a more comprehensive description of convertible virtual currencies to date, see Financial Crimes Enforcement Network (FinCEN) Guidance on the Application of FinCEN’s Regulations to Persons Administering, Exchanging, or Using Virtual Currencies (FIN-2013-G001, March 18, 2013).
IRS Announcement 2002-18 presented some clarity to frequent flier miles as those too are considered a closed virtual currency since they can be redeemed for goods or services, but never for fiat.
IRS Announcement 2002-18
Most major airlines offer frequent flyer programs under which passengers
accumulate miles for each flight. Individuals may also earn frequent flyer miles
or other promotional benefits, for example, through rental cars or hotels. These
promotional benefits may generally be exchanged for upgraded seating, free
travel, discounted travel, travel-related services, or other services or benefits.
Questions have been raised concerning the taxability of frequent flyer miles or
other promotional items that are received as the result of business travel and
used for personal purposes. There are numerous technical and administrative
issues relating to these benefits on which no official guidance has been provided,
including issues relating to the timing and valuation of income inclusions and the
basis for identifying personal use benefits attributable to business (or official)
expenditures versus those attributable to personal expenditures. Because of
these unresolved issues, the IRS has not pursued a tax enforcement program
with respect to promotional benefits such as frequent flyer miles.
Consistent with prior practice, the IRS will not assert that any taxpayer has
understated his federal tax liability by reason of the receipt or personal use of
frequent flyer miles or other in-kind promotional benefits attributable to the
taxpayer’s business or official travel. Any future guidance on the taxability of
these benefits will be applied prospectively.
This relief does not apply to travel or other promotional benefits that are
converted to cash, to compensation that is paid in the form of travel or other
promotional benefits, or in other circumstances where these benefits are used for
tax avoidance purposes.
Since the promotional account credits that were offered were a closed virtual currency due to their inability to be traded for fiat, the federal government does not deem them taxable and they can be seen as completely valueless. Even though the business may try and argue that they are used to "pay" for a bill or services, these credits are seen nothing more as a nontaxable rebate to the IRS.