It doesn't matter what presumably patented feature Jolla has stepped on, that's just an excuse to bring the company into litigation. It may just be that they use - OMG! - roundish icons.
The problem is going through those litigations is expensive, and while all those companies you mentioned have plenty of money to burn (and a resident attorney dept. to feed), Jolla cannot afford any of that.
Europe is a relatively safer ground as such nonsensical litigation requests are not granted that easily.