What's your opinion on "premium Drupal modules"?

We had this discussion 4 years ago. Why bring it up again now? Because several big CodeCanyon projects are coming to Drupal soon and I think it will have an impact. One of them is Slider Revolution. Slider Revolution is an "All-purpose Slide Displaying Solution that allows for showing almost any kind of content with highly customizable transitions, effects and custom animations". With nearly 60.000 sales at 18 USD it's the second most popular Wordpress plugin on codecanyon. The number of sites using this module is much greater because hundreds of premium Wordpress themes ship with the slider built into the theme. 

To refresh our memories here are some quotes from 2011:

the DrupalAppStore that killed drupal

..one thing that open source doesn't do a good job with: building teams of people with complementary skills to make sure that the software is a good experience for the customer. Why? Because there is no customer. Oh sure, hundreds of thousands of people use my software and they consider themselves customers, but ultimately they are not. Why? The definition of a customer involves, among other things, providing a revenue stream.

The pay-per-copy business model just doesn't work very well, practically, unless you have the completely artificial system of copyright restrictions to prop it up. (Physical objects have a natural scarcity that makes pay-per-copy vastly more practical.) When you're dealing with copyleft software, it works even more poorly.

Sometimes I keep wondering why on almost every drupaller comment I read on the net is against making money on selling modules but it is OK to sell themes?

If themer can get away / circumvent GPL by licensing their css/images/js in different license than GPL why can't module developer create a separate php class api that doesn't touch any of drupal api and license it with commercial license?

With respect to the question "How" this last commenter was on to something. Large projects on CodeCanyon protect themselves against redistribution by having a functional code library that can work independently from the CMS integration. If there is any open source lawyer reading this I would love to hear comments on GPL compatibility of this structure.

My opinion

As a (premium) Drupal themes developer I have a special interest in development of these plugins: they provide great value to clients of my premium theme. For only around 100 USD I can buy an extended license for a module that cost the developer hundreds of man-hours to develop. By including several plugins into my theme which costs 48 USD, a lot of value is added instantly. In general I have a positive attitude to CodeCanyon developers joining the Drupal community. However, there will be some modules that are good for Drupal and others that could be bad for the Drupal ecosystem. 

For example, me and several other Drupal developers have been working on improving Bootstrap+Views integration through the views_bootstrap module: https://www.drupal.org/node/2203111. In the meantime, some guy on CodeCanyon seems to have all our problems figured out already and he is selling a very sleek Views+Bootstrap module on CodeCanyon. The code he sells is all Drupal-integrated programming. As far as I understand GPL this means that all his code is also GPL. So what can we do, is it legal to copy his code into the views_bootstrap module? Is it compliant with the rules and code of conduct on Drupal.org? Is it morally OK?