The potential implications of this are huge. Not just open sourcing, but imagine easily decompiling and modifying proprietary apps to fix bugs or add features. This could be a huge unlock, especially for long dead programs.
For legal reasons I bet this will become blocked behavior in major models.
I've never seen a law forbidding decompiling programs.
But, some programs forbid to decompile applications by the license agreement. Further, you still don't have any right on this source code. It depends on the license...
A mere decompilation or general reverse engineering should be fine in many if not most jurisdictions [1]. But it is a whole different matter to make use of any results from doing so.
Using an LLM (or any technique) to decompile proprietary code is not clean room design. Declaring the results "open source" is deception and theft, which undermines the free open source software movement.
Only if you use the decompiled code. But if one team uses decompiled code to write up a spec, then another team writes an implementation based on that spec, then that could be considered clean room design. In this case, the decompiler would merely be a tool for reverse engineering.
It is true that at least some jurisdictions do also explicitly allow for reverse engineering to achieve interoperability, but I don't know if such provision is widespread.
For legal reasons I bet this will become blocked behavior in major models.