I'm going to talk in proper internet speech because this is an interesting topic that I've put some thought into.
The thing about competitive fighters online is that they allow people to use *every* advantage available to them, and one of those advantages is poor sportsmanship. In person and at tournaments and things like that, being a dick has consequences. It makes the person you're playing with like you less, and in turn all of the people around you. HOWEVER, it cannot be denied that getting under someone's skin and making them angry throws off their game. Everyone that plays games can recall at least one time where they got pissed and started making mistakes and poor decisions they normally wouldn't.
Online, getting under someone's skin is a huge part of the game. Taunting, using frowned upon tactics, and playing like a jerk has no consequence, because your opponent will likely never meet you, or possibly ever play with you again. But it still causes people to play poorly, which makes it a tool. I know that I try and win games by playing smart, but there have definitely been times where I've been a jerk, because it makes it easier to win. The simple act of taunting in SSF4 throws off so many people who play that taunting after a command throw is worth it; you'll get more openings that way, and they'll come attack you in ways that they shouldn't or wouldn't otherwise.
In short, because there is no social consequence for being a jerk online, it's a tool that cannot be overlooked. Does this make it acceptable? Not really. But it is unavoidable that most people will end up that way over time. The interesting exception to this rule is playing against people that you know or have planned to play against. In this setting people are usually polite, and when they're not it doesn't have the same effect it would in an anonymous setting.