I assumed that it either had something to do with filesystem patents[0] or the vague patent claims they made on early Android vendors. UI design didn't even cross my mind - I didn't even know those were
patentable, and Apple's foray into "look and feel" lawsuits was something Microsoft adamantly fought
against. Hell, if I've heard correctly[1], Microsoft was the reason why those early UI design flourishes like faux-3D highlights made their way into CDE and Motif.
Given that Miguel de Icaza already is calling BS on this I'm starting to doubt the veracity of any of this.
[0] Microsoft still claims ownership over ExFAT, for example
[1] It was vaguely mentioned somewhere in NCommander's very, very long "Installing every version of IBM OS/2" stream