The license is really baffling:

"a) Neither the Work nor any of its individual components, in Original or Modified Versions, may be (i) sold by itself; (ii) used for website or app generators; (iii) used to create templates, themes, and plugins for sale."

It doesn't disallow using them for personal or commercial use by developers but I admit that most of what they've done must rely on the Bootstrap APIs so I have doubts there's much that can be copyrighted by them in what's there.