I think we're going to see a lot of this in the future, where someone creates a free version of a Wordpress plugin based on a commercially available plugin. But what's the legality of it? Mjewel? Got an opinion? For example, there's now a free SEOPressor clone, and Daniel Tan is already apparently threatening to sue. But what possible grounds could he have?
On what grounds is he suing? Copywrite? Trademark? Passing off? Patent? How are these "clones" doing what they're doing? For a working example, lets take a straight forward example that captures 404 errors and redirects people to a selected "appropriate" page with a 302 response. If someone decides this is a good idea and they want to copy it, they use their own code to hook into the search facility on a 404 error and 302 to the most relevant result then the only way there could be a problem with this is if there is a patent on the idea. Your certainly not breaking any IP simply by re-engineering the idea. Obviously if your brand it to look the same as the original etc then you open up new issues. Likewise, if you just repackaged the code rather than writing it from scratch then you have issues. Patents on software depend on where you are, in the USA they are full available and are an explicit subset. In the UK they are much more difficult to obtain because our laws are slightly outdated and there is no specific considerations for software.
That was my thoughts, Astaroth. Thanks for the lucid explanation. AFAIK, Tan seems to think his plugin is in some way 'copyright'. Presumably he doesn't understand the concept of copyright. While his source code may be copyright, a plugin performing the same services written from scratch is just ... tough luck, and welcome to the world of competition.