People who operate on a website for extremist behavior should be punished under the law. I don't have a problem with someone reading the information to become more informed. The problem lies in the people who post and encourage other people to participate in these extremist ideas. We should punish these people as terrorists.
People that use extremist websites should not be punished because they should be free to express what they believe. They may be doing research about the website, or they may be curious about what the content of the website is about. There is no way to tell what their intention is on the website.
Users of extremist websites haven't done anything wrong unless they infringe upon the rights of others at some point. Extremism is bad, but not the point that we should censor websites just because they make us uncomfortable. Extremists using websites aren't dangerous until they pick up a gun or bomb and start pushing buttons.
Users of extremist websites should not be punished unless they have actually committed a crime. Simply reading or contributing to a website is a right protected by the first amendment. Law enforcement officials should monitor extremist websites, but they are perfectly legal. It is important to keep our freedom of speech.
Individuals that visit and use extremist websites shouldn't be punished under the law unless they actually use such resources to commit crimes against others. Just visiting those websites shouldn't result in any sort of criminal penalty, though. A person can visit and use whatever site they want as long as they don't commit crimes afterward.