According to an APWU article:

APWU initiated a national dispute in 2006 over the Postal Service’s revisions to the AS-805 handbook governing Information Security. The revised handbook restricted employees from bringing personal information resources (e.g. laptops, notebooks, PDAs, handheld computers and USB port devices such as flash memory sticks) into postal facilities. The union expressed concerned that the new restrictions may adversely impact the union’s ability to perform its duties.

Recently, the Postal Service settled the dispute with Unions by inserting new language in the AS-805 Handbook:

These policies do not change the rights or responsibilities of either management or the unions pursuant to Articles 17 and 31 of the various collective bargaining agreements or the National Labor Relations Act, as amended. These revisions do not bar the unions from using their own portable devices and media for processing information that is relevant for collective bargaining and/or grievance processing, including information provided by management pursuant to Articles 17 and 31 of the collective bargaining agreement or the National Labor Relations Act. There is no change to policy concerning restricted access to the Postal Service Intranet.ents or the National Labor Relations Act, as amended. These revisions do not bar the unions from using their own portable devices and media for processing information that is relevant for collective bargaining and/or grievance processing, including information provided by management pursuant to Articles 17 and 31 of the collective bargaining agreement or the National Labor Relations Act. There is no change to policy concerning restricted access to the Postal Service Intranet.