I had the same rejection at DT last week for photos of graffiti spray painted on the side of a train. In this case, the issue was copyright/trademark infringement and they asked that I try to obtain a property release.
Imagine trying to get a property release from the "artists" who vandalized the train.
I'm going to resubmit them with a note explaining the "artwork" was created by an act of vandalism, and ask if it's protected under copyright/trademark law. Seems to me it would make sense that anything created during a criminal act can't be copyrighted or trademarked...but I could be wrong.
Anyway...the only place where there was an announced vigilance about property releases was SXP. Haven't heard anything about other sites doing the same. In fact, SS doesn't even provide a property release on their site.
The DC monuments aren't copyrighted or trademarked. They are all in the public domain. Try sending an e-mail to Alex @ 123rf, point that out, and ask for a second review. They are pretty good about reconsidering.