I think it was used wrong. CMIIW, but you only need to separate clauses with a comma if the subordinate clause comes before the main clause, so this: We squint at the sun, because it's too bright

Should've been this: We squint at the sun because it's too bright

But if the subordinate clause comes before main, then a comma is needed: Because it's too bright, we squint at the sun

However, the above mistake is not a mistake in German and a few other languages. If you translate the above sentence in German, it looks like this: Wir blinzeln in die Sonne, weil sie zu hell ist (yes, i use Google Translate)