However, many software firms have no issue with selling the development of a feature to customer A, and then selling a license for said feature to customer B. This is the same situation as selling the skills acquired at employer A to employer B. Why is it ok for employers to do this, but not for employees?
It is much more than that. An employee sells not skills, but time of his life, the only one he's got. A company can make million sales, an employee can't work a million jobs.
