Well, no orthopedic surgeon would be doing a stent or bypass, either. Orthopedic surgeons are also trained to recognize, diagnose, and treat LE manifestations of systemic diseases, as well as trauma. Orthopedics is one of the most highly sought-after specialties for MD graduates, one of the most competitive residencies to obtain, and one of the higher-paying.
Granted, these are not necessarily legitimate reasons to prevent other health professions from joining the field, but you can probably understand how bitter people would feel if after getting into MD school, working hard in medical school to get a competitive residency in orthopedics, somehow miraculously matching, and spending 5+ years in allopathic residency, there was a podiatrist who's allowed to do the same thing you are after 4 years of podiatry school and a 3 year residency. The same is the case with ophthalmology. They're concerned about the well-being of their field, since optometrists are pushing to have the same rights, responsibilities, and scope of practice (in essence). Sure there's some ego involved, some finances involved, some patient safety/care issues involved, but to be honest, it just doesn't seem right.
It would be acceptable and morally defensible to sacrifice these areas of MD practice to other health professionals if it were pretty clear that patient care would be better or at least as good with other benefits than the status quo (MD care), but, again - separate is inherently unequal, and the fact remains that significant differences exist between the education and training (we'll leave "practice" out of it) of an MD and DPM, and currently it doesn't seem to be in favor of the DPM.