Suppose I spend a few hours making a necklace and Ms. Smith decides she wants to buy it. I agree to sell it to her for $20 and she agrees to pay me $20. Both Ms. Smith and I are happy with the transaction. Wouldn't it be ridiculous for someone else to block this voluntary exchange or force either of us to change our agreed upon price? Maybe you happen to think that Ms. Smith should have paid me $25 for the necklace. Or maybe you think the necklace was really only worth $15. So what? It is, quite literally, none of your business.
It is irrelevant whether you personally agree with the price that the two parties believe is fair. You are not a party to the contract -- where do you obtain the ability or the right to decide its terms?
*Editor's note: In case it wasn't apparent to the reader, the above is an analogy. It illustrates a principle in the most basic terms. Apply it to any voluntary exchange in which an unaffected third party, say for example - the government, is able to determine the terms of the contract, even though both parties are fully satisfied with the transaction and no individual's rights are violated.