Where the heck do you find any notion of everything being GNU GPL??? Before buying it states that it is Open Source - which is correct. But it states nowhere under which license. Open Source does not mean it's free!!!
If it were - then you could not sell single site license vs multi site license - because under GPL you could use as many instances as you like.
So no - Krum cannot use the Stripe implementation - and he knows that pretty well.
Or maybe you need to learn English?
"Buy the software and it's all yours, including access to the source code. That means if your developer needs to review the s2Member source code or help you with an issue you're having, they have access to all of the source code. Also, there are NO recurring charges, NO limit on the number of members you can build-up over time. Plus you get free lifetime access to updates; i.e., future versions of the software."
It does not state anywhere you are allowed to sell or reuse the code. It means you have access to the source code - no more. That's like saying - you can download this MP3 without copyright protection - it does not mean you can download this MP3 and do with it whatever you like. The MP3/song is still copyrighted, but it's distributed without protection measures.
Also I won't go down to the same level of language you are using here - this is unprofessional to the max (and not the first time).
S2member Pro is a constituted work - consisting of several parts - you seem to be unable to grasp that concept (which is well possible under GNU GPL)
Copyright: © 2013 WebSharks, Inc. (coded in the USA)
= This Software is Comprised of Two Parts =
- All PHP code (and integrated HTML) in this software have been released under a GPL license; just like WordPress® itself. You can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation.
Unless you have our prior written consent, you must NOT directly or indirectly license, sub-license, sell, resell, or provide for free; part (2) of this software; or make an offer to do any of these things. All of these things are strictly prohibited with part (2) of this software.