How to prevent intentional over-estimation in user stories?

Question

I am asking this from a purely hypothetical standpoint.

According to the Sprint Planning Meeting section in the Scrum Guide: "The number of items selected from the Product Backlog for the Sprint is solely up to the Development Team. Only the Development Team can assess what it can accomplish over the upcoming Sprint."

Questions:

1. What is there to stop the development team from over-estimating intentionally the time it takes to complete user stories?
2. If there is a long-term contact with the product owner and development team, should this be defined in the contract as some sort of violation or breach of contract?
3. Who would be responsible for discovering such a violation?
4. How could the product owner prove that such a violation was taking place?

Thank you for reading.