IRS Publication 590-A at pages 8-9 makes it clear that the annual limit extends to nondeductible as well as deductible contributions to a traditional IRA. As to why? I would suggest that, even though there is no upfront deduction, there is still a tax benefit in years after the nondeductible contribution is made. You're not required to pay tax on growth of the investment until it is taken out of the IRA. Assuming investment in mutual funds, you have to pay tax on required annual income distributions if that investment is not in an IRA. Having no limit on nondeductible contributions would place no limit on the ability to defer tax on such investment income. HTH
