The narrow answer to your question is that the so-called "fair trade laws" enacted at the state level in the 1930s were all repealed by federal law in 1975. See the topic "retail price maintenance" in Wikapedia. The fair trade laws were originally enacted not to protect manufacturers from discounters but to protect small, independent store owners from large chains (which came into existence in the 1930s) that could get lower prices from suppliers and undercut the independents on price.
Until 2007, retail price maintenance imposed privately by manufacturers (not by law) was automatically illegal. The rule was you couldn't control price after you sold the item to someone else. That approach was supplanted in a 2007 Supreme Court decision holding that each situation had to be evaluated on its facts to determine if there was an economically reasonable basis for the price maintenance. Retail price maintenance may be illegal in certain circumstances, but is not automatically so.
Until 2007, retail price maintenance imposed privately by manufacturers (not by law) was automatically illegal. The rule was you couldn't control price after you sold the item to someone else. That approach was supplanted in a 2007 Supreme Court decision holding that each situation had to be evaluated on its facts to determine if there was an economically reasonable basis for the price maintenance. Retail price maintenance may be illegal in certain circumstances, but is not automatically so.