Why did TurboTax enter 100,000 on form 8915-E line 4 if column B was not more than 100,000 and the instructions say to leave it blank?

That's just the prefilled default amount on a blank form.  Ignore it

https://www.irs.gov/pub/irs-pdf/f8915e.pdf 

As stated on the IRS Form 8915-E for line 4 -

Add lines 1 through 3 in columns (a) and (b). Complete column (c)
if line 4, column (b), is more than $100,000. Otherwise, leave column (c)
blank

The amount of $100,000 is already entered on Line 4 of the Form 8915-E.

Go to this IRS website for Form 8915-E - https://www.irs.gov/pub/irs-pdf/f8915e.pdf

Doesn't make sense that they would fill that box if the instructions say leave it blank, but thanks for your answer.  Glad I asked. 🙂

you can send your comments on inconsistencies or confusion in the forms and instruction here.

https://www.irs.gov/forms-pubs/comment-on-tax-forms-and-publications

We all hope, Form 8915-E will never appear again.