These are not decisions made by a jury of objective bureaucrats- I believe if a company wants labeling for home use it represents another expensive round of hurtles that are often redundant- but sometimes it is about the high concentration of the pesticide and the manufacturer doesn’t choose to invest in the residential market by offering more dilute formulations.
If you do a search for scientific study of health consequences of OP’s from normal usage, the data is contradictory and controversial. Most cases of acute poisonings have come from suicide attempts. 90% of overall poisonings in the U.S. come from drugs but the percentage from pesticides seems impossible to find, indicating a relatively small number.
For me, the best indication that OP’s are not excessively dangerous is the farmers health study by the U.S. Dept. of Health following the health records of farmers and their spouses for over about 25 years now. A lot of these farmers are older people who were highly exposed to all forms of OP’s before there was much concern (don’t forget that DDT was routinely used on humans to treat lice in the '50’s). As I’ve often mentioned here, these farmers are much healthier, live much longer and get less cancer than the overall populations in their states.
This doesn’t mean that the pesticide application aspect of their life-style is not detrimental to their health, or doesn’t increase certain types of cancer, but it is a clear indication that these materials are not as dangerous to human health as the general public assumes.
Imidan is actually not considered very effective on plant bugs (or Avaunt)- at least according to Cornell. It certainly didn’t seem to control tarnished plant bug on my property. Neonics and pyrethroids are more effective.