Unfair dismissal

I was getting jobs through Staff Unlimited for a number of years. Then when I got sick at Pembury tours and got fired, they no longer wanted me on their database. They told me I could not be trusted and re**** upon!! They told me they removed my CV from their database. Why don't people looking for work get the better of the doubt. Why do the employers always get the "approval " and the employee is always wrong??