There are places where one should mute the phone, like cinemas, offices, cafes, etc. Other than that, I'd like to hear when someone is trying to reach me.
An US thing?
Here in the Netherlands it would seen as very peculiar to have a ringtone enabled.
Here in Germany (actual residence), Portugal and Spain (as my home culture), Italy and Greece (relatives), it is still quite common, unless people are at work, or at places like cinema and theater.
The point is that it's not only seen as odd in the US. That there are counterexamples where it's normal doesn't change that.