Do they not?

UNOOSA, Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, Art. IX [1]:

"States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination..."

Note that State parties are responsible for the actions of their nationals later in the paragraph.

Is the above wrong? I admit to not being a space person.

Edit: [1] http://www.unoosa.org/pdf/publications/STSPACE11E.pdf