I did a little research on the laws that seem to deal with this.
Here is the relevant DC regulations I found regarding the posting of signs:
24 DCMR
108.4 Any sign, advertisement, or poster that does not relate to the sale of goods or services may be affixed on public lampposts or appurtenances of a lamppost subject to the restrictions set forth in this section.
"appurtenances" means support systems to the lamp posts such as the electric box they are posting on. The press conference was on a city street and the electric box was serving the lamp posts on that street. The restrictions for this are as follows:
24 DCMR
108.5 A sign, advertisement, or poster shall not be affixed for more than sixty (60) days, except for the following:
(a) Signs, advertisements, and posters of individuals seeking political office in the District who have met the requirements of the DC Campaign Finance Reform and Conflict of Interest Act; and
(b) Signs designed to aid in neighborhood protection from crime shall be exempt from the sixty (60) day time period.
24 DCMR
108.6 Political campaign literature materials shall be removed no later than thirty (30) days following the general election.
24 DCMR
108.7 Each sign, advertisement, or poster shall contain the date upon which it was initially affixed to the lamppost.
24 DCMR
108.8 Each sign, advertisement, or poster shall be affixed securely to avoid being torn or disengaged by normal weather conditions.
24 DCMR
108.9 Signs, advertisements, and posters shall not be affixed by adhesives that prevent their complete removal from the fixture, or that do damage to the fixture.
24 DCMR
108.10 No more than three (3) versions or copies of each sign, advertisement, or poster shall be affixed on one (1) side of the street within one (1) block.
24 DCMR
108.11 Within twenty-four (24) hours of posting each sign, adverstisement, or poster, two (2) copies of the material shall be filed with an agent of the District of Columbia so designated by the Mayor. The filing shall include the name, address, and telephone number of the originator of the sign, advertisement, or poster.
24 DCMR
108.12 For purposes of this section, a "public lamppost" is any public post erected for the purpose of supporting electric wires.
Please note that even the regestration with the city about the postings can be done within 24 hours of posting. Please also not that "public lamppost" is defined as any post erected for the purposes of supporting electric wires. This includes telephone polls, street lights, and traffic lights and all their associated "appurtenances."
These cops did not know the law if they accused them of defacing public property. The citizens told them they beleived they were acting legally and they disregarded them, physically intervened and arrested them anyway. This looks bad for the cops if you ask me.