The way I unterstand it, at least in European practice, the code itself is copyrighted, as in, the actual text, not the concept behing it. Decompiled source is not the same as original source, it goes through rounds of compiler optimisations and the process of reversing that includes a lot of guesswork. So, in practice, decompiling and working on the decompiled source was at least tolerated, probably because it would have been a grey area at worst.
The way I unterstand it, at least in European practice, the code itself is copyrighted, as in, the actual text, not the concept behing it. Decompiled source is not the same as original source, it goes through rounds of compiler optimisations and the process of reversing that includes a lot of guesswork. So, in practice, decompiling and working on the decompiled source was at least tolerated, probably because it would have been a grey area at worst.