The Court ruled that just because an agreement barred further exercise of the terminationright, it did not run afoul of the statute which does not permit an initial agreement that prospectively eliminates the terminationright.
Although they may have made what now looks like a bad deal, it did not entitle them to termination under the Copyright Act because that bad deal was made with the knowledge and power of the terminationright as it stood in 1992.
The Shuster heirs would not have had their initial terminationright until 1994 because of the intricacies of the previous 1909 Act, to which Superman is subject.
Afraid that grantees like DC would reap the benefit of these 19 years without fair compensation to authors, Congress included a terminationright, allowing authors to take back the rights in their works before the extended period took effect.