"""
It is compatible with the popular GPL licenses, so users of scikit-image can choose to make their code available under the GPL. However, unlike the GPL, it does not *require* users to do so (BSD is not a so-called *copyleft* license), so scikit-image can be readily used in closed-source, commercial environments.
"""

Or some variant thereof. =) Here, we must make clear to people unfamiliar with all of these:
1 - what "compatible with" means.
2 - what "copyleft" means.

In fact I would not have known what copyleft meant without the context.