It's not about power dynamics, it's about liberty. And ultimately, liberty is about autonomy. Having the freedom to do what you choose if you so need to. Free as in freedom right? Stallman was always talking about liberty.
Potayto, potahto. As an example outside of computing, slavery — the antithesis of liberty/autonomy — is the manifestation of an extremely skewed power dynamic.
Yes, it is to protect liberty to do what you will with the software/source.
It is also to make sure that the developers of said software will be in a much more difficult situation when it comes to abusing the users of their software. Stallman talks about this as well.
Liberty for whom? Most of humanity is not economically privileged enough or digitally literate enough to utilise the available digital liberty, let alone able to know that it exists.