Well lets put it this way... If you look at the otherway. If you are not the legal holder of any copyrights
Then you also now that you as a starting point are not allowed to do anything with any code.
Then of cause the whole ide of open source is that we can obtain that right from the owner or the developer.
This is a common practis. Most of the time you do get an approval or already in the license
There are statement that you are allowed to modify, change or use snippets of the code for your own work.
But sometimes you get a "no" then we just have to respect this.
It's not just about the legal aspect this is also about moral and respect for those who spend years of their free time
To provide the users with their work.
Next step is that all developers start putting the code in to .so files and then everything will be closed source.
Regards
Seagen