Just because thousands of HN readers are intelligent engineers, founders, scientists, marketers, etc. doesn't make them legal experts, or political experts.
You assume that "freedom to contract" means that both parties enter with equal knowledge and equal bargaining power. In an environment with high unemployment (like we were not to long ago) it could mean that employees would jump at any job they were offered, out of necessity, even if it came with a nasty non-compete.
Government regulators can look at an overall picture and see that, statistically, in great numbers, there are overarching problems with the non-competes, like their being used with employees who don't have trade secrets.