Each agency uses their own standards. These standars are based on different peerspectives. The USPS looks at who is more likely to steal mail; all types of felons and many misdomenents might be a higher risk to them. The TSA looks at who's a security threat. In that they are bound by the FARs, which specify several different felonies (including murder, rape, sedition, air piracy...) and a very few misdomeners. The TSA has asked for a lot, and believe it or not, are likely often told to pound sand because the regulations governing them tend to be rather specific. Since the USPS operates as a looser entity, permitted to establish their own policies, they have more flexibility as well. It's not a question of risk to one thing or another, but one of perspectives and restrictions on policies.