If MOS people think it's OK to use code/icons/whatever from AROS (why reinvent the wheel, after all?), then they should also think it's OK for AmigaOS4 to use code/icons/whatever from AmigaOS3
The problem is that AROS code was written from scratch, in C and is more or less hardware independent while amigaos3 code is targetted for amigas and contains some asm.