Deal With Unfair Job Termination Clauses

Unbeknowst to many, there is always a clause in the termination letters of employees that forbids them to work in companies or businesses that practice the same kind of trade as their former places of employ.
Just what kind of ruling is that? Just why do companies and the public allow that? And just what kind of a right does a company have to demand that its former employees desist from working in a rival company?
To date, most companies have been doing just what does not appear as being ethically right. Too often than not, they insist on their departing employees not to work in a rival company engaged in the same kind of business for fear that trade secrets would be squirreled away. For instance, a security guard or IT programmer who after being dismissed are sometimes not allowed to commence employed in the same industry for a period of time.
What if a dismissed employee is offered the same kind of a job in a rival company that offers him or her, a higher salary and better perks? Must he or she accept it or oblige his former employer who could have dismissed arbitrarily? Will there be penalties if he accepts it?
The Independent was not able to get answers from human resource lawyers but as it appears there have been clauses in termination contracts forbidding former employees for working in rival companies.
Such clauses do not just retard job growth and job mobility but are an unnecessary check on the choices available to an ex-employee.
The real hard question is, should former employees simply ignore such moratoriums or heed them?