I believe the principle is, sans any anything more formal, CAVEAT EMPTOR. "Scamware" is certainly not the only example of this.

now for the regulatory agencies on commerce
Hopefully, doing things of greater significance with the limited resources at their disposal. Maybe if we legalized drugs we could afford a "war on scammers" and even the lowliest potheads of the consumer scam world would have something to fear. I doubt this is in the interest of the cocaine kingpins of the consumer scam world however, so don't hold your breath.

send me 5 dollars and i will send you a copper bust of a president
Okay, so you have lost your retirement fund?

People pay more than that for a lottery ticket. What do you expect? If I buy something at the app store for $1.99 and I don't like it, can I return that? How about a chocolate bar?

i mean why do a scam when there are many books with free demos on how to do whatever ? instead of scamming the general public.
Because the individuals that make up "the general public" have different perspectives WRT what $$ they can afford to invest in something vs. the time they have to research it. If I thot I might save myself 4 hours of research by gambling a few bucks, I might do it, on the principle that it is worth the risk. If it does not work out, I wasted 1/2 hour and a few dollars, and now (if I'm that serious) must perhaps do the research. What's the difference, in terms of resources, for a member of the general public to spend 4 1/2 hours + $5 VS. just 4 hours? Obviously the second is preferable, but if there is a chance the first choice might actually mean only 1/2 hour + $5, a lot of people will go for it.

You could say, well we could eliminate the risk but we will have to raise your taxes to pay for enforcement, most people will accept the risk.