Most download taxes that are based on "what" rather than "how much" you download are unconstitutional, for the same reason a tax targeting pornography is unconstitutional:
It is a content-based tax. Content-based taxes discriminate against certain kinds of (disfavored) content, and First Amendment law is well established on the point that content-based taxes are a no-no.
You can tax goods and services, but your taxes must be structured in such a way that they do not discriminate against content. You may tax paper and DVD stock, or you may tax all sales, but you can not tax just books but not movies, or just magazines but not books, or only pornographic movies and magazines but not Time and Newsweek.
In the context of a download tax, the only possibly permissible way it could work is by-the-bit (or other unit of arbitrary measure). You have to tax email exactly the same as web pages exactly the same as software downloads exactly the same as movies from Netflix. To do anything else introduces an impermissible content-based discrimination (along with the impossibility of measuring what you're taxing).
And while lawmakers may not want to target pornography because it might "legitimize" the product, they fail to realize that adult content purveyors have decades of experience fighting this sort of nonsense in jurisdictions all over the nation, and on this point, they consistently win.