"then at a minimum you should verify regularly that the code has not changed (you have to hope that you are looking at the same code that your users see)"
Who exactly is the "you" in the above statement and who pays the "you" money to fix this and keep on top of it on an ongoing basis? And for how long?
In the physical world the different between ideal and practical can be described by my experience with production machinery. The machinery came with guards to protect the operators from getting their hands caught or cut off. The guards also came with switches to prevent the machines from running when the covers were taken off. But what would happen is the operators would want to oil or tweak the machines so they would take the covers off and disable the sensors so that the machine would run bare. Of course you would tell them not to do this, but they would still forget to put the covers back on or be lazy quite often and there was little you could do about it. You had production to get done under deadline and weren't likely to fire someone even though you knew there was a small safety risk in doing this type of thing (older machines of course came with no safety guards at all, operators just had to be careful at their own peril. (And good operators were impossible to find anyway so once you had someone they became a primadonna ..