Even in a group that doesn't care about copyright at all, it'd be tacky to copy someone else's code and not credit them, or to pretend it's all your own work.
Might be true. Not familiar with Amiga scene but the following actions were always mostly ok in the European C64 scene, provided credit was given to original author:
* Re-using music in whole
* Re-using fonts
* SOMETIMES re-using small parts of graphics
It always was not ok and considered "stealing" to re-use complete demo routines though for example.
People did get away (reputation wise) with re-using code routines for tools, though, provided credits where given and the re-released tool somehow improved on the original.
Another thing which many people disliked for example was re-using "instruments" from music, even when melodies changed. While the players were mostly open tooling, specific instruments/sounds for those players still had to be crafted first and I witnessed a couple of conflicts when people reused the instruments.
Nothing was written in stone, though. A concept such like licenses did not exist.
So those plundering the digital high seas would not only remove copy protection and write an "intro" to both boast about it and show their coding skills, they'd also steal music from commercial games.
And they were the OGs: the demoscene only came after the work (a bit shady of a work but a work nonetheless) of these pioneers.
Another one that in my opinion distinguishes the scene behaviour from the classical copyright system is that the latter allows the copyright holder to use the state violence to squeeze out damage payment from copyright infringers.
Applying this kind of violence seems not to be accepted in the scene.