There are other rectangular phones which Apple is not suing. Why is that? By investigating this question, you may shed light on whether rectangles alone is the issue or not.
Because they don't sue everybody? It's widely understood that Apple is avoiding bringing Google to court, for example, for strategic reasons. And some "infringers" are just too small to sue. It's a cost benefit calculation, not an admission that Samsung is the only offender. And Apple certainly has sued plenty of other companies for similarly frivolous reasons.