Pirate has just too many negative connotations. Would someone create a party called the "Communist Party" in a democracy today? Even if their intentions would seem much different than the "old communists", I think it would have too much trouble getting off the ground (in any democratic country, not just US, where it's impossible for any third party to get off the ground).
This is also why I dislike the names darknet or darkmail. Prosecutors in US are bad enough as they are [1] (and remember Cameron Ortiz? [2]). Imagine them having to explain what a "pirate browser" or a "dark net" is to a jury. They'd say that they clearly named it that way because their intent was evil from the beginning (even if the tool may be technologically "neutral"), and they intend it for doing bad stuff with it. And I imagine the jury, who wouldn't know any better, would tend to buy it.
And I also agree completely that normal people may try to stay away from having something called "Pirate Browser" on their desktop.
I think I understand why people usually choose such names for their projects. They want to rally their supporters faster, people who really get what it's about, but I think it would do just as well based on what it does, and not because of its name, with that early adopter crowd, since they will understand what it's about and what it does.
[1] - http://www.nytimes.com/2014/01/05/opinion/sunday/rampant-pro...
[2] - https://petitions.whitehouse.gov/petition/remove-united-stat...