Why do you call it Open Source? The plugin itself does not call itself Open Source, and clearly links to another project for those people who want an Open Source program.
The code being free does not mean usage is free. Qt’s code is free to look at, but you still need to pay them for certain things. Licenses apply to the code, not what it does.
> Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files