Libraries do have to make curatorial choices about what to stock, both physically and digitally. So it’s not surprising a lot of libraries don’t stock a book that was never widely published or sold many copies, regardless of political connotations. And as controversial books go, I don’t think the Turner Diaries is one of the ones that necessarily merits wider availability for “historical interest”. I’ve never read it, maybe you have, but from I understand it’s not even a particularly interesting book it itself, only for it’s connection to some right-wing terrorists like McVeigh. The fact it never sold much sort of indicates not many people want to read it, a good reason not to carry it!
And for ebooks in particular, it’s worth remembering most libraries (including Brooklyn) outsource their digital catalog to a third party provider. So their curatorial choices are potentially limited by that as well. Again, self published book never widely published or sold: not surprising to me that the major ebook providers for libraries might not have it.
As a litmus test against your litmus test, it’s worth noting that the BK library does carry other controversial books like Mein Kampf, physically and digitally.
All of this to say that while politics is always going to influence curatorial choices of “merit”, I think suggesting that the Brooklyn library “banned” The Turner Diaries merely due to its absence from the catalog is a bit of a stretched conclusion.