The Obama administration decided arbitrarily not to enforce Federal law. If the laws remain on the books, they should be enforced. If we as a country don't want those laws to be enforced, then our elected representatives in Congress should amend or repeal those laws. But until that happens, the Justice Department is not empowered to pick and choose which laws to enforce, and which they will ignore, particularly if the choice rests on politically corrupt decisions.
The issue is that marijuana is classified as a schedule 1 substance, lumping it in with all of the hard narcotics that fall under the Controlled Substance Act, which isn't getting repealed any time soon. This is so because they consider marijuana to be highly addictive, and of no medical use, an opinion that (recent data suggest) a majority of Americans do not hold. This classification makes it impossible to use medical marijuana (nevermind recreational) in states that have legalized it, since federal law allows for prosecution regardless of state law. One alternative is reclassifying marijuana as something other than a schedule 1 substance, but this may not be popular in some states. The other is to do exactly what was done (in a bi-partisan manner mind you) during the Obama administration, which is to advise prosecutors not to pursue charges against users who are abiding by state law, provided they meet certain standards. That was the goal of the Rohrabacher-Farr amendment, which essentially removed any federal funding for the justice department to use in preventing states from enacting medical marijuana laws. That amendment was signed into law in 2014 and is still on the books.
This feels like a good middle ground, where state law can matter, and a federal law that would be extremely hard to change is still relevant in states that have no legalized medical/recreational marijuana. Sessions' rescinding of the memorandum advising prosecutors not to pursue charges against (state) legal users is a slap in the face to this law, and to state enacted marijuana laws.