Laws in probably every other country are the same in this regard. If you enter the country and do any work under the wrong visa, it is an improper entry.
Nobody expects you to say "yes, I want to defraud old people" but if evidence comes up that you intended to do illegal stuff they now have a simple case of you lying to the immigration officer.
Similarly to how you are supposed to pay taxes on income of illegal activities, if they find stacks of cash in your house it becomes easier to convict you.
They are weird laws but have their uses (good or bad)
A direct question like that is seen to be useful as part of a broader set of questions, due to the emotional effect on the interviewee, depending on the "interviewing technique" that the agency is using.
Every time I visited the US I noted a complete absence of common sense.