Earlier quoted context omitted.
Why unethical? Person has obligations to themselfs and their family, not to some corporation they do not even work for.
I don't have a firm answer to this, it's more of a feeling that "this is wrong", but here are some possible arguments. I'm not sure they are the best possible arguments, but they're a quick first pass at making the argument. It is good for society as a whole if people release their work publicly/open source. People will only release their work publicly/open source if it doesn't substantially hurt their potential earn…
I see opensource project that is missing usefull features and users who need them.
Why is "redirecting stream of money" towards me bad? Maybe I run orphanage and can use money more ethically...