It is my opinion that "lawyers" are keen to remind us that if someone makes no attempt at protecting their intellectual property they have actually forfeited some rights to claim how it gets used.
So, if you share your code and want some control over it... add some form of license at the top of the code and to any related texts. I believe that the nature of the "requirement" about derived work is that you include "credit headings" from your reference. If they claimed original work.. then that's all. If the work you are deriving yours from is also derived from other sources... it would already be noted in the credits headings you are adding to your source.
If you "find" code with no license... you technically can do with it what you please. Good behavior dictates that you at least say where "your" code is derived from lest we start to hear claims of "foul play"
The nature of the Arduino and its community lends itself to open source code and sharing of ideas. The CC license seems well suited to what we do as a whole.
Original work may be used in commercial solutions. Seems fair.
If the Arduino community stopped acting like an open source/shared ideas community and instead started abusing each other and all around started behaving badly... well, I guess you can see my point about where that would lead.
"We" and the Arduino design are "successful" mainly because of the community it created.
Below is an example of some recent work I did. Eventhough I scrapped a lot of the original work, I felt compelled to give proper credit as my work was derived from the original authors work
/* Filename: outgauge.h
Compiler: wxDevCpp Version 126.96.36.199 (MINGW32)
Project: OutGauge Bus
Original Author: Vladimir Kadlec http://www.fi.muni.cz/~xkadlec
Revision: Serial Interface and Packet Update revisions - Pete Willard
Serial Modification for microcontroller support: Pete Willard
Now sends commands out serial ports to an external Microcontroller
A lot of the original code was scrapped, Sorry Vladimir.
I just didn't want to figure out WINSOCK UDP code by myself. This is
therefore, a very modified derivitive of the original GI.EXE
ORIGINAL: Gear Indicator is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
Gear Indicator is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with Gear Indicator; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA