Reverse engineering proprietary software isn't necessarily illegal. From the 9th circuit court decision in "Oracle v Accolade" (2021):

We conclude that where disassembly is the only way to gain access to the ideas and functional elements embodied in a copyrighted computer program and where there is a legitimate reason for seeking such access, disassembly is a fair use of the copyrighted work, as a matter of law.

Disclaimers:

• IANAL.
• A prohibition on reverse engineering can certainly be included in an EULA, so in some cases, reverse engineering a piece of software can constitute breach of contract.