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.