What happen is that, usually, the employee is not satisfied with the work, pay or just wants to change job descriptions and start applying to interesting positions. If the company advertising the position has any sort of relationship with the current company the employee is working at, it will notify the employee superiors causing serious repercussions internally. Note that none of this is described in any non-compete clauses. Those are unspoken pacts between companies.
I think companies believe that making their lives miserable in the process will reduce the resigning rate, but it only creates a culture of distrust. Because the alternative, making employees lives better, paying better salaries and implementing quality of life programs takes effort and costs money in the short run.
In the long run however, I believe everybody would benefit from a culture of cooperation, not only executives.