Sorry. Reread the statute C-A-R-E-F-U-L-L-Y. "The following ITEMS of income SHALL BE TREATED AS SOURCES" makes it pretty clear that for the purposes of that section of the code, "items" and "sources" are semantically identical--and that such is the legislative intent.
And claiming that trying to understand the meaning and legal import of the term "operative sections which require the determination of taxable income" is comparable to Clintonian weaselling about the word "is" constitutes a reach in my book.
You are explicitly NOT trying to understand--you are deliberately acting as stupid as possible. Problem is, it only works ONCE.
Oh give me a flippin' break, Poohbah!! Take your own advice, for crying out loud!
The following items of gross income shall be treated as income from sources within the United States
The following ITEMS shall be treated AS INCOME, not "AS SOURCES."