When did postal employees become non-government jobs and why?

They didn't. They are still government jobs.

They are a government agency in some areas such as employee rights but they are self contained and don't receive tax dollars to run the USPS. They rely completely on the cost of postage and other services that they provide to pay their employees and buy equipment etc to keep them running. They are also mandated by the federal government that they are not allowed to make a profit. Therefore every penny they receive is spent on the equipment and salaries to keep things running. That is one of the reasons our postage in this country is so cheap. Try mailing a post card from another country and you will quickly find that our postal service is a real bargain. Don't get me wrong I still avoid sending letters etc for emails as they are faster and even cheaper than the cost of a stamp and the paper to write on.

http://en.wikipedia.org/wiki/United_States_Postal_Service

there still federal, look at the way they work and the long slow lines

They are government employees...However, the US Post office is their own government entity.