Contributed $8,000 to IRA and recharaterized all to Roth later. Why TurboTax calculated my recharaterization as Taxable IRA distribution?

The result suggests that you reported a Roth conversion, not a recharacterization.  If the Form 1099-R has code 1, 2 o 7, the financial institution processed a Roth conversion.

were you trying to do a backdoor Roth and expecting it to be nontaxable.  if so then you probably input something wrong in the 1099-R or nondeductible contribution interviews in TT.  

see detailed steps here

https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-ira-conversion/L7gGPjKVY_US_en_US

in particular you need to make sure the IRA contribution is input as a non-deductible contribution; and for the 1099-R there are questions about "what did you do with the money", ensure you select "I did a combination of rolling over, converting, or cashing out the money" and there is question for how much was rolled over to a Roth.

check the outcome on Form 8606 in your return - this is where the basis and rollover is tracked and tax calculated.