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btw, that's fine to use intro.js < v2.0.0 without purchasing commercial license for commercial projects (apps, plugins, themes, etc.)

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I'm not sure if you're aware, but the terms of the MIT License apply to commercial as well as non-commercial use. That means it would be fine to use intro.js commercially without a commercial licence.

There's some confusion here about dual licensing projects. Maybe an internet lawyer can comment. The most successful project that I know of that uses dual license is qt which offers a GPL license for open source and a $500+ per month license without restrictions for commercial. Some people say you can just link dynamically and include instructions for how to relink yourself, but if that's true why would anyone provide qt any money, especially at $500per month or whatever rates they charge.

In this instance though Intro.js is covered under MIT not GPL so the Qt restrictions don't appply.

Qt uses the LGPL, not GPL: http://www.qt.io/FAQ/ - you're right about the linking stuff. But that means opening stuff in ways that is probably undesired, e.g. embedded devices.

In the case of this MIT-licensed work, IANAL but there seems to be no way to require users to buy the commercial license because the MIT license already allows this usage.

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