Earlier quoted context omitted.
I would agree these are system-wide less efficient, but shouldn't paid "garden leave" be legal if parties agree upfront?
Well if all companies are doing it, employees will have no option other than to agree. This is the current case with non compete afaik.
If all employees rejected them, companies would have no option but to disband them.
The issue is that when one enters into a non-compete without equivalent compensation the government loses potential tax revenue. Income tax is maximized when workers are working most productively, so there is a strong incentive for the government to keep workers working productively. Similar reason for why we have minimum wage.
If non-competes provide sufficient "garden leave" then the tax revenue should not be impacted, so there is no reason for the government to care. The productivity is still captured. The government isn't in the business of spotting bad deals for individuals – you are quite free to make all kinds of boneheaded financial decisions – only bad deals for itself.
Granted, execution and enforcement can be a challenge. Say you enter into a non-compete with "garden leave" of $1. Is that sufficient compensation with respect to the intent of allowing "garden leave"? Now you need a tribunal to make the call and all the complexities to go along with it. And so, when drafting laws, it may be more practical to prohibit "garden leave" as well.