It's not inside the mosque. It is a shared building.
YOU were barred because you're a disruptive POS.
Under 39 CFR § 232.1 (Conduct on postal property):
• § 232.1(e) (Disturbances) prohibits disorderly conduct, loud/unusual noise, impeding entry/exit, obstructing entrances/foyers/corridors/parking, or any conduct that tends to impede or disturb postal employees in their duties or the public while conducting business.
• § 232.1(d) requires everyone to comply with official signs and the directions of security personnel or other authorized individuals (including postal staff).
Violations can result in removal, a fine (under 18 U.S.C. § 3571), and/or up to 30 days imprisonment. These rules apply to the portions of the building under USPS control (the leased retail space). News photography is generally allowed in public lobbies unless restricted by signs or staff directions, but it does not protect disruptive behavior.