Final answer:
The IRS might have decided not to tax the value of frequent flier miles earned on airline tickets purchased for business use because they are considered a fringe benefit and business-related expenses are generally not taxed.
Step-by-step explanation:
The reason why the IRS might have decided not to tax the value of frequent flier miles earned on airline tickets purchased for business use is d) Business-related expenses are generally not taxed. The IRS considers frequent flier miles earned on business trips to be a form of non-taxable fringe benefit provided by the employer. This is because they are seen as a reward for the business travel and not as actual income.