Why did my employer put me on probation?

My job is very hard and frought with danger. I constantly have to go into dangerous neighborhoods. I have been accosted, I have witnessed gun battles. Yet, when I made a big clerical error, none of my dedication was considered and I was put on probation with a strong threat of termination. I have worked for this company about 4 years and my work is at minimum satisfactory. Why was my work record not considered? Why is the culture in this country (USA) so poor that my job is in jeopardy for one mistake?