That's an interesting change in the law. Postdoc stipends are not "earned income" because you are not being paid for performing "work" (regardless of what it feels like when the alarm goes off Monday morning), you are being trained and educated. This means you are not subject to self-employment tax (social security and medicare). Normally this also means you don't have "compensation" for purposes of contributing to an IRA, but you are correct the tax code now allows postdoc stipends to be considered "compensation" for purposes of an IRA.
In that case I would suspect that the program has not been fully updated for all new tax law changes. Try again in January. Note that Turbotax can only post changes once the IRS had made the changes to their forms and instructions, and then it usually takes a few more weeks since the IRS also has to specifically approve of the change in the tax program. Sometimes, tax law changes are not approved until February or even March.
