Why in America obvioulsy unfavarouble contracts for one side are not banned?

Because it's up to the parties to the contract what they will sign- it's a free country and people are free to sign whatever they want. It's also a bit subjective as to what "unfavourable" means, and if someone can take the costs, they might want to offer a generous contract for some other benefit.

It's only if the contract was signed under duress, misrepresentation or false pretences, or unconscionable conduct, in which case it wasn't freely entered into, that it might be invalid. Apart from that, it would have to be an illusory promise for it to be banned, or something similar.