Prisoners are also unable to become smokejumpers or hot shots because they not only have a criminal record, but don't get the same training such as the EMT requirement, yet they're put in harm's way. Democracy Now covered this topic extensively.
It's often a CYA and public perception issue: org X receives application from convict, considers fallout if convict does something bad, realizes it'll become a PR nightmare and that they'll be torn to shreds for permitting the hire. The risk is never outweighed by the benefit, in their eyes.
Many (most?) western countries do not publish convict names unless it's very serious, such as murder. There's no reason a typical employer, let alone the public, should know you served time in the past.