As I pointed out, most optometrists tend to set up shop near the food court in the mall, and thus you will not likely encounter them in a clinical setting where they might be confused with an ophthalmologist.
Optometrists being in clinical settings with MDs/Dos is a relatively low probability event. I know that we live in a society that is obsessed with legislation based upon rare events and all, but, that doesn't change the fact that there is very little possibility an optometrist can be confused with an MD/DO in a setting where it is actually important. Optometrists aren't going to be working in hospitals and they can't set up private practice as anything but optometrists. It seems the only case of "confusion" presented thus far is your one anecdotal observation. And it wasn't even confusion really. Parents tell school psychs to shove it all the time--I'm not sure the "I'm a doctor" bit really mattered. Even if it did, your n=1. Come back to us when you have enough data to do some statistics. There simply is no epidemic of optometrists being mistaken in clinical settings for being anything other than optometrists. Ditto chiropractors, podiatrists, dentists, veterinarians, naturopaths, Chinese medicine docs, PhDs in basic sciences, epidemiologists, or anyone else. It doesn't happen.
There was a PhD student in sociology at my alma mater who earlier this year passed herself off in a health care town hall meeting as a "doctor" to both a Congress critter and reporters. Since pretending to be a doctor in that state is a felony offense, I don't believe she is in any danger of being a doctor of sociology or anything else at this point. So yeah, every once in a blue moon some idiot pretends to be a medical doctor. It either has no impact on society (as in your single observation), or they get busted by the law and they wind up paying some hefty fines, do jail time, etc.
So yes, I suppose there is some very, very, very small danger some unscrupulous optometrist can decide he wants to move from next door to the food court to inside the university medical center and treat patients. I'm just not convinced this hypothetical individual would progress very far in such an endeavor before having his hypothetical ass handed to him by a number of hypothetical entities that stand in the way of just such things happening.