Why does TurboTax demand that I upgrade from free file in order to add a standard 1099-INT from a redeemed us treasury Series I Bond?

[DoninGA]

How much interest was received from the Series I Bond that you redeemed?  If the interest earned is $1,500 or more then a Schedule B is required and the Free online edition does not support Schedule B.  You would need to upgrade to the Deluxe edition.

[ee-ea]

There are more reasons for a Schedule B than @DoninGA  mentioned.

https://www.irs.gov/forms-pubs/about-schedule-b-form-1040

[AnnetteB6]

TurboTax is committed to filing your return completely and accurately.  If you are being asked to upgrade from one version to another based on something that you entered into your return, then it is because the current version you are using does not support the form or calculation that you need for your return to be complete and accurate.  

It is always your choice to include or omit income or deductions that you have in order to use a certain version of the program.  However, you may be contacted by the IRS if your return is incomplete.  You are the one preparing and filing the tax return by using the TurboTax program.  

To learn more, see the following TurboTax article:

Why do I have to upgrade from Free Edition?

@segoviafan