That is because of the legal system in the United States which is separated into Federal and State Governments. What is legal on the Federal level may not be legal on the State level. When it comes to 80% lowers, the ATF regulates the rules on them Federally, but States control the regulations for transfers of ownership of firearms apart from federal requirements. State governments control what must be done in order to make their receiver blanks legal.
Some States like Oregon do not allow even the serialization of 80% lowers, effectively making even the sale of these illegal. However, states like California will allow 80% lowers to be serialized before being made into a fire i.e. milling out the trigger pocket.
On the Federal level, 80% lowers are completely legal, and not by any sort of pushing of the line or “loophole.” In fact, it is actually backed by the ATF that the uncompleted receiver blanks being sold are legal. There are multiple letters issued by the ATF which cover the legalities of these incomplete lowers. This is especially true in the letters that regulate the building and assembly of 80% lowers.
Since they are not 100% complete firearms, they do not legally require a federal background check, however, states that require serialization may require that they be put on the state registry of firearms once serialization is complete.
The best way to be 100% current on the laws regarding 80% lowers cannot be found on the private internet forums or the news media outlets, but in your states Department of Justice, as well as the the ATF website, which contain the actual documents in reference to the laws you are most concerned about. It is safest to approach 80% lowers with the knowledge that you are acting within the parameters set by the federal and state laws that regulate them. Going to the source is how you will remain safe from any hassle from the State or even the ATF itself.