Signing and accepting jurisdiction are two different things. Clinton's contemporaneous statement (from Wikipedia):
The United States should have the chance to observe and
assess the functioning of the court, over time, before
choosing to become subject to its jurisdiction. Given these
concerns, I will not, and do not recommend that my
successor, submit the treaty to the Senate for advice and
consent until our fundamental concerns are satisfied.
Nonetheless, signature is the right action to take at this
point. I believe that a properly constituted and structured
International Criminal Court would make a profound
contribution in deterring egregious human rights abuses
worldwide, and that signature increases the chances for
productive discussions with other governments to advance
these goals in the months and years ahead.
Clinton thought that being a signatory with contingent intent would lend the ICC legitimacy even if the U.S. would never submit itself to its jurisdiction, and even if it was obvious to everybody it wouldn't submit itself to its jurisdiction. It's like the opposite of plausible deniability--plausible ratification. According to his logic, the fiction mattered. To others it was pointless.
