I believe that is because they require a fission component in order to set of the fusion component. If they could obtain fusion without the fission detonator, it would be clean from what I've read.
Without the fission component they would be cleaner, but not clean.
All the existing thermonuclear bombs use fusion reactions between various isotopes of hydrogen and lithium and all those reactions produce an intense flux of neutrons, which will generate large quantities of radioactive isotopes in the surrounding environment.
The same problem of the neutrons exist with most projects of getting energy from fusion, which also produce radioactive waste, even if less than fission reactors.
