As mentioned in the game rules already, every player is allowed to play on a single account only. Since this might be a bit uncommon for sandbox MMOs, I'll try to note down some reasons for this decision here.
Reputation is important:
If you get a well known pirate or even a griefer, you have to accept the consequences. Other players may hunt you, and you can't simply hide behind a second account then.
Spies are annoying:
Many alliances in other sandbox MMOs have problems with spy alt accounts, wich lowers trust to new players. It also could continuously damage the amount of new players. If you can't create an infinite number of new spy accounts, this problem solves itself hopefully.
Keeping things simple:
Instead of managing multiple accounts at once, we try to keep it as simple as possible for you (the end users). You have a single account for the website, the forum and the game, and you shouldn't worry about other players getting an advantage by owning more accounts than you.
We will enforce this rule at all cost, with all available means and hard punishments for rule violations. Even the attempt to create a second account can get you banned permanently.