Public Park Designated Copyrighted Space
If a sculpture is in a public place, you can take a photograph of it. (Assuming US law applies and excepting Military Reservations)
You cannot, however, commercially exploit that picture without permission of the copyright owner.
This is all simple copyright law.
If you are being harassed by any law enforcement officer, please ask them why the Executive Branch is becoming involved in a civil matter.
Alteratively, you can explain that you are gathering photographs for the news outlet "slashdot.org." Furthermore, explain that a copyrighted work, in a public place, can be reproduced for Fair-Use reasons, including the gathering and reporting of news.