What is the ruling on forums that require swearing by Allah's Most Exalted Name to read their topics, along with invoking curses upon those who violate the conditions, and is one who swears with the intention of acquiring knowledge considered sinful?

Merely clicking on a section icon does not constitute an oath because an oath is only binding through verbal utterance or writing with intention. Websites setting conditions for entry and requiring an oath to that effect is permissible, unless the knowledge on the website is something that must be disseminated, in which case it might be considered concealing knowledge.