This question already has an answer here:
How "legal" is it to read and edit .exe files, .dll files, and other compiled source files? Decompilation is taking the compiled code of a program, often minified and obfuscated, and trying to get the original source code back as it was written, or at least get it in a form a person could read. There are various reasons a person or company would make its code harder to read while compiling it, but the 2 I've seen most often have to do with saving resources, like space, RAM, power, etc., and making it so the code is harder to read so the user can not see what it does as easily. As we are a community about questioning the practices of reverse engineering, I think it's safe to say most of us don't like the second reason. I am asking for facts here, though, not opinions.
Because this question could get very broad, I will limit this to laws in particular countries and rules of various influential or popular companies, but info about generalities is also welcome. I am definitely not asking about morals/ethics, although I know those will certainly come up.
There was already a question about the legality of reverse engineering in general, but I'm talking only about decompilation here. As the answers to this question stated, decompilation is one type of reverse engineering. As far as I could tell, the other question didn't get answers about decompilation specifically, though you are free to provide these yourself. One answer did say the question was too broad, but perhaps this question will have a better scope.