Copyright

and indeed OsCommerce copyight. ZC is released under version 2.0 of the GPL that states

c) If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)

There is a lot in this section of the forums that states that copyright signs cannot be removed. That is incorrect. Copyright including the ZC team can remove the original OsCommerce copyright signs as long as the original work is made available (as that would contain the original copyright) and as long as it is made clear that the work is a derivation. This section of the GPL was written to avoid having to add copyright for every person that had ever written a part of the program in what was called the "OpenBSD advertising clause" and can be read about here:

http://www.gnu.org/philosophy/bsd.html

What people commonly misread is

You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty;

as you must keep the copyright notices in, but you'll notice I have highlighted the term "verbatim copies" e.g. if the original ZC is being sent out around, it must contain all of the copyright notices, if the version is not the original there is no requirement in the GPL apart from an attribution "originally based on" and the requirement

You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change

The general theory being that the ZC team can then see what was changed in their work although they may very well have to pay for a download, afterwards the work would still be GPL and they could then distribute it, they would then be require to make the version they downloaded available too, but could incorporate any changes into their work and leave out the copyright, but would have to date it and state the original source.

Bit long winded and most of the modules that I have downloaded from the add-ins section although GPL released do fall foul of the prominent notices and dating clause of the GPL v 2.0 which is a bit of an issue mainly because probably no-one knows exactly which bit was modified from which file at what point. For my personal use on my first installed shop, I modified several define files but as I'm not distributing them, that's not a problem.

It's debatable as to whether changing the text in a define file is covered by the GPL, since the changing of text does not necessarily change the execution of "the program", but on the flip side it certainly comes under the scope of a derivative work in terms of copyright laws and the GPL would cover that file under the clause 0

The "Program", below, refers to any such program or work, and a "work based on the Program" means either the Program or any derivative work under copyright law

and then there is another contradiction here:

The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.

Zen Cart regards the output from the program as independent of the program otherwise one would end up where every piece of text and image from every shop would then be GPL covered (that would be a minefield).

Summarising:
a) copyright notices do not have to be kept,
b) changes have to be obvious and dated
c) if Zen cart were to apply "interaction" to the display of the zen cat copyright's e.g. visiting a website or logging onto to admin, then they would be obliged to display osCommerce's copyright on all pages as well as insisting all shop owners did, plus the copyright of any contributed module

Changing text in define files would appear by precedent to have been excluded from the GPL by the Zen Cart team, although then one runs into a possible issue of whether ZC is not "the program" doing the output, but the webserver is, which could probably keep the lawyers busy for some time.