"I was under the impression you can buy firearms legally without background checks at all in much of the USA"

I'm not sure if this is legitimate or if you're trolling. The regulations on background checks can vary depending on the state. Even in states where private sales are legal, you would still be prohibited.

Granted enforcing that prohibition comes down to people following the laws, and there's nothing physically forcing them to do so (whether requiring a background check or not), merely punishment after the fact.

If you buy person-to-person, yes, in most states. However, just because a felon (or a marijuana user, both are prohibited from owning guns) can buy a gun like that, doesn't mean they won't get in trouble for it if they ever get caught.

Edit: form 4473 question 11e asks if you are an unlawful user of drugs such as marijuana and reminds you that it is still federally illegal. You will fill out this form if you purchase a gun from a licensed dealer. Lying on this form is itself a felony.