A lot of families in USA view this as a norm. While it is not in the majority of the world. I'll imagine there is history behind it.
Follow up (precursory?) question:
Is it normal for Americans to wear shoes indoors?
Just to clarify, are you comparing the US to a particular culture? What parts of the US wear shoes indoors as the norm? Do most parts of the US allow guests to wear shoes indoors (i.e. socializing, dinners, BBQ, etc.)?
I'm not sure what defines the norm, but wearing shoes indoors for something like a dinner party is very different from someone wearing shoes indoors in their own home 90% of the time. I'm inclined to think the former is more common than the latter, but my experiences might not reflect the norm. I think it would be difficult to pinpoint the norm for a country as large as the US however.