That's probably a form of supplier induced demand. As long as there will be lots of programmers willing to dedicate their whole day (and night) time to work, there is no reason for the employer to accept any extra-effort with uncertain extra-outcomes by following uncommon employment policies.

So unless a growing fraction of developers starts recognizing the benefits of part-time employment and start demanding it from their employers, it is unlikely that these change their policy for those reasons stated in the other posts.