I can't find much information about the religious significance except that different colours traditionally marked different statuses (wife, widow etc.) and the forehead is meant to represent wisdom.
And bindis have become a fashion accessory in Indian culture, so what's wrong with someone wearing it as a fashion accessory? It's not like it's a purely religious symbol any more, or like this lady wore a bindi with a more traditional meaning.
@Idsbabe Other than being annoying, what's wrong with cultural appropriation? Many people complain that "people from other cultures don't integrate" in the UK, but that works both ways surely? I can see how a white atheist calling their son Mohammed might offend Muslims, but I can't see what's wrong with seeing things in other cultures, thinking "that's beautiful/nice/inspiring" and adopting it into your own life in some way is a bad thing.