While both practices are problematic, bribes are categorically worse. A flat fee only gatekeeps people who don't have enough money, whereas bribes gatekeep people who are ethical.

Furthermore, an author who pays a fee will still have their paper will still go through a (hopefully rigorous) review process involving input from independent peer reviewers as well as the editor. Paying the journal a fee does not directly and intentionally subvert the review process, whereas bribing the editor does.

When the money goes to the employee of the institution providing the benefit instead of the institution itself.