It is a good and right thing that you're giving thought to moral issues, and I salute you for that.
However, I strongly suspect that the situation you describe is more urban legend than fact.
For one thing, welding is a highly skilled operation requiring specialized equipment, and it would likely cost more than the shoes were worth to get something like that done. I'm having trouble visualizing how it could even be done without damaging the shoe.
For another thing, anyone who was inclined to weld things to his footwear could do it to ordinary steel-toed work boots by cutting away some of the leather on the toe.
Golf cleats, spiked racing shoes or cowboy boots with spurs could all be used in evil ways similar to what you're worried about. However, that's not why they're made or what they're intended for, so moral responsibility for harmful actions would rest with the perpetrator, not the maker of the items or the merchant who sold them.
If you were selling something that was actually designed and created for the primary purpose of causing bodily harm, with no other redeeming uses, you'd have much more cause for moral concern.
In the case of steel-toed boots, it occurs to me that having the metal protection on the outside rather than the inside could be quite practical, because it would be easier to keep your feet warm on a really cold day.
If this particular footwear style makes you feel too icky, that's your call. But I don't think you'd be crossing any moral lines if you sold them off to at least recover the cost of the goods.