In my understanding, put a slider on a website template is one (valid) thing, it results in another type of product. But when buy a slider and wrap it in a 'wordpress interface', you create a direct competitor to the slider.
I'm not saying that your understanding is wrong, it makes sense, i'm just stating what i think
And in the end i think you are right, i will not offer extended license any more.
One more time, thanks for your help.