It's usually against EULA, but that has nothing to do with the law, no?
But is it illegal to provide tools for decompilation? As in shooting people is illegal, but selling guns is not.
Shooting people is not illegal in the US -- I'm not sure this is the best analogy or there will be huge limitations when discussing decompilation efforts.
Plenty of people are shot or killed lawfully with firearms.
I doubt that will happen for a tool with an arbitrary usecase of assisting in research however some projects related to reverse-engineering have been censored under the DMCA takedown regime.