The expression in English is “set foot” not step foot. That’s racist, you Anglo-centric bigot. You can use both.
No, one either steps or sets foot. On does not "step" one's foot.
It is an error born of illiteracy. People hear the words incorrectly and if they don't read very much they don't catch the error. Somebody likened it to saying, "for all intensive purposes".
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