If you leave line 7 blank, it has the same affect as entering a $0 on it as far as the calculations are concerned. You can contact an agent at TurboTax to ask about it, but you won't be able to speak with a programmer, and they would be the only ones who could explain to you why the form works the way it does.
As far as I can tell, the form does properly calculate your alternative minimum tax however. Also, TurboTax guarantees the accuracy of your tax calculations, so if an issue were to arise because of the way the form 6251 was completed on your tax return, TurboTax would reimburse you for any economic injury you suffered.
You can use this link to contact TurbTax:
https://support.turbotax.intuit.com/contact/
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