The basic premise of your statement is certainly correct and indisputable, however it is still logically flawed.
The lives of individual animals are governed by "natural selection" and selective pressures. Working under a number of assumptions, it is easy to assume that hognoses will undoubtably eat toads that are easiest to catch. Of course one of the broad assumptions with predators is that they consume the weak and inferior. When you go out to catch toads to feed a pet snake, you are completely bypassing the natural selection process put on the toads AND your snake. Most people don't want to mess with catching a lot of little toads, it is human nature to go for the biggest and most energetic. Of course those individuals tend to be of breeding status and most notably females. And of course your snake is completely exempted from any of the selective pressures that are put on wild snakes. Your snake has no threat from predators, nutrition, parasites (that is of course, you are one of the few people that apparently believe in prophylactic treatment of parasites for hognoses) or anything else you can think of. It is just sitting there are gobbling up animals that actually have to fight to survive. It is simply not going to die if it is not strong enough to catch its next meal, instead it has the very animals it may not be able to capture itself, hand fed to it.
And it seems like toads have even more problems stacked against them, everything from pollution of their water and their food sources, to the fear of predation from people who allow their cats to roam outside. Also, many people make the predictable assumption that an animal that is common is not threatened with extinction. There are many examples of animals that were so ubiquitous it was inconceivable for them to dwindle and number. But there are equally a number of examples that demonstrate that some species can not exist at numbers below common. Just because there are dozens of toads around your house, does not mean that the population can still flourish with only a few individuals. With the typical wide population fluctuations between years of amphibians, it is very easy, even for a toad, to go locally extinct. What would your untrained hognose eat then?
Lastly, I would like to comment on the disappointingly one-sided views many herpetoculturists have. Even though grouping amphibians and reptiles into the science of herpetology is unnatural, I would like to think that people with an interest in one group would be equally concerned about the other. I don't understand how people can have pets like an eastern hognose (that is ironically also exhibiting signs of decline) and be completely unconcerned about amphibians or any other taxa. The important thing that all herpetoculturists should have ingrained in them is a sense of compassion for all wildlife, and especially awareness of issues concerning the very animal they choose to keep.
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"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)