How to effectively get a post-doc position
I would recommend both applying for the postdoc positions advertised, and also writing to professors with research groups that you are interested to join. 

Applying for a postdoc is similar to any other job application. You are responsible for sending out the applications or reaching out the future employers. A pro-active approach will invariably be beneficial to the applicant. 

As an applicant, I would not recommend worrying about the fairness of the jobs advertised. Just apply.