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.
It's usually against EULA, but that has nothing to do with the law, no?