Midwood Yid: I expect you won’t like the explanation (I don’t think it’s very principled mayself), but here goes:
Every library must make choices about what materials it offers. If has a budget and space limitations that limit the number of books it can have – it can’t have every book ever printed, so it has to make some choices about what to offer. It may make the choice whether or not to offer its patrons certain materials based on a variety of factors. For example, a public library in Boro Park might choose to spend a significant amount of money of materials that would interest the Jewish population and to not spend money on entertainment DVDs.
BUT, a government funded library cannot choose which materials to offer based on the viewpoint expressed by those materials. Thus, library could not categorically refuse to buy any materials advocating communism, a two-State solution to the Middle East conflict, or the Democratic Party.
For the same reason, filtering internet content is highly problematic because it typically is directed at censoring a certain point of view (i.e., pornography (yes, that is considered a viewpoint about various “issues”).
Thus, while a library could decide not to buy computers because it thinks buying books is a better use of its budget, or could decide to buy computers but to not offer internet access (again for budgetary reasons), it cannot buy the computers, offer the internet access, but limit access to only certain points of view or topics. Imagine the library in backwoods KKK country deciding to block access to any website that says anything positive about Jews.
Obviously, as Torah-observant Jews, we believe that their is a qualitative difference between accessing YWN and pornography. But for good reason, in the eyes of the law, the government cannot distinguish between them. If the library cannot limit patrons’ internet access to only online editions of Mein Kamf, it also can’t limit access to only non-pornographic material.