I don't see the contradiction. Amazon are afraid of their employees having more power. Their success was built upon an open market, but they are trying to prevent others from benefiting from this.
A "Proprietary Information and Assignment Agreement" is about protecting proprietary information. It is illegal in all 50 states to use trade-secret information at a new employer. See Anthony Levandowski.
A Non-compete is about labor market, wage, and competition suppression.