The purpose of the provision is actually to deter licensors from attempting to use copyleft-next as a mechanism for promoting proprietary license sales in the manner we have seen in the past with other copyleft licenses. I don't expect it to have some "gotcha" effect on some entrepreneur who failed to read the license. The point is to prevent such use of the license in the first place.
In this sense referring to it as a 'poison pill', while sort of appealing, may be misleading. It's effectively a "keep away from this license if that's what you want to use copyleft for" sign.