Are they allowed? If not, why not?

People create alts all the time. As for in-game:

• It's legal for someone to have two accounts in the same room so long as it isn't an official server where they can earn tc/QI per win.

So if you are like a tenth dan black belt, you could make a new one to get tc on beginner servers or something?

Yes, however it IS frowned upon. The beginner servers have belt restrictions in order to keep out people who have more experience than new players. Nothing is stopping you from doing that, but it's generally not a nice thing to do.

Ok I'll only do it when I'm desperate. Thanks!

Easy TC doesn't justify ruining the game for new people. You're allowed to play in the beginner servers as long as everyone's happy with it. If people complain and you get caught, you'll just get kicked. Besides, there are better ways to get TC than playing in the beginner servers.

when you are banned you can make a new acc right?

Of course you can, make sure you don't do the same thing why you got banned.

^ may pinapatamaan ka diyan lol