For teh lulz - The reason why anyone does anything:
It is a tradition formed as early as the birth of the first ever online discussion forum. Its always best to secure a safe location to which you can fall back anytime some senior member or mod strongly disagrees with your point. Always try to keep a small amount of humor or sarcasm in your posts to reflect your casual take on the matter.
When someone disagrees with arguments you can't counter, simply reply with a "Don't tell me you didn't know i was joking" comment and you'll be successful in making the other person feel like a fool for his actions.
Admin's word is absolute:
Always try to raise a point to which the admin will most certainly agree. Since the Administrator is the most powerful force on the forum, his word is considered final. Unless of course when he doesn't agree, thats when you'll have to adopt a different strategy.
Trying to ignore his post and continuing on with the discussion at hand is the easiest and most commonly used tactic, or better yet, get into a serious argument with another person to get the attention of the suckers away from the what the Administrator just said.
If you're lucky, the admin will forget about what he might have said and will focus on resolving this fight between you and the other person. Now that you've caused considerable confusion and chaos in the thread, you can easily slip away like the troll you really are.