Agree with you that "set foot" is by far the preferred construction and that in this usage "step foot" is clearly incorrect, but "step" as a verb has both transitive and intransitive uses:
v.i.
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to move in steps.
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to walk, esp. for a few strides or a short distance: Step over to the counter.
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to move with measured steps, as in a dance.
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to go briskly or fast, as a horse.
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to come easily and naturally, as if by a step of the foot: to step into a fortune.
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to put the foot down; tread: Don't step on the grass.
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to press with the foot, as on a lever or spring, in order to operate some mechanism.
v.t.
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to take (a step, pace, stride, etc.).
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to go through or perform the steps of (a dance).
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to move or set (the foot) in taking a step.
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to measure (a distance, ground, etc.) by steps (sometimes fol. by off or out).
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to make or arrange in the manner of a series of steps.
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to fix (a mast) in its step.
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step down,
a. to lower or decrease by degrees.
b. to relinquish one's authority or control; resign.
- step in, to become involved; intervene.
I, for instance, have stepped several masts.
Agree with you that "set foot" is by far the preferred construction and that in this usage "step foot" is clearly incorrect, but "step" as a verb has both transitive and intransitive uses:
v.i. 17. to move in steps. 18. to walk, esp. for a few strides or a short distance: Step over to the counter. 19. to move with measured steps, as in a dance. 20. to go briskly or fast, as a horse. 21. to come easily and naturally, as if by a step of the foot: to step into a fortune. 22. to put the foot down; tread: Don't step on the grass. 23. to press with the foot, as on a lever or spring, in order to operate some mechanism. v.t. 24. to take (a step, pace, stride, etc.). 25. to go through or perform the steps of (a dance). 26. to move or set (the foot) in taking a step. 27. to measure (a distance, ground, etc.) by steps (sometimes fol. by off or out). 28. to make or arrange in the manner of a series of steps. 29. to fix (a mast) in its step. 30. step down, a. to lower or decrease by degrees. b. to relinquish one's authority or control; resign. 31. step in, to become involved; intervene.
I, for instance, have stepped several masts.