I think all the points are very valid.
I work in a corporate environment, have work in small business and even owned a few myself.
It is more about adequate management than over controlling adults. If the proper policies are in place, employees are educated and trained on all policies, have proper employer/employee communications and involvement, you have a higher morale and better commitment from the employee's also.
If the policy says no personal social media on company computers is a no no during specific or all times, for specific positions, etc, then is management issue to have a proper relationship with their employee's that they have the ability to lead and management that environment, if can't they are not a good manager, and in my experiences, many are not. Now there may be some positions that take special considerations, but over all just out right blocking is not the proper work environment for any access, except to block potentially harmful sites from malware, etc.
Except for having a policy that prevents discussions about an employer or work related content, no one has the right to barr anyone from being associated and using any media or saying what one wants without any repercussions in their private lives and/or on their own time using their own resources period.