you cannot even "shameless copy" from something that is asking the world to copy from it

yes you can if you do not honor the license that the original was released under.

that's why we use FOSS licenses in the first place

some FOSS licenses are intended to insure that the code remains open even after modification. that's used by anyone for anything under the same license.

Yes, fair, I agree; follow the license, that's the single both legal and social expectation.