This memo states federal policy, though how long it will last is anyone's guess. One important thing to note is that there was no official change in regulations. The memorandum ends with the caveat that
this memorandum is intended solely as a guide to the exercise of
investigative and prosecutorial discretion. This memorandum does not
alter in any way the Department's authority to enforce federal law,
including federal laws relating to marijuana, regardless of state law.
Neither the guidance herein nor any state or local law provides a
legal defense to a violation of federal law, including any civil or
criminal violations of the CSA. Even in jurisdictions with strong and
effective regulatory systems, evidence that particular conduct
threatens federal priorities will subject that person or entity to
federal enforcement action. This memorandum is not intended, does not,
and may not be relied upon to create any rights, substantive or
procedural, enforceable at law by any party in any matter civil or
criminal. It applies prospectively to the exercise of prosecutorial
discretion in future cases and does not provide defendants or subjects
of enforcement action with a basis for reconsideration of any pending
civil action or criminal prosecution. Finally, nothing herein
precludes investigation or prosecution, even in the absence of any one
of the factors listed above, in particular circumstances where
investigation and prosecution otherwise serves an important federal
A half a year ago, it might have been politically impossible to prosecute on federal charges, but it was still technically possible. The policy is currently under review by DOJ. Since possession of marijuana is a general crime (i.e. not limited to "on federal land"), it is a prosecutable offense.
There is no reconcilation necessary per se: a person can be prosecuted at state or federal levels, so if a state lacks a mechanism for prosecuting a person, there is still the federal mechanism. Local police, however, are not obligated to enforce federal law.
More generally, though, if state and federal law actually contradict each other, the Supremacy Clause says that federal law take precedence. Such a federal law would of course have to be within the proper powers of the federal government, and as held in Gonzales v. Raich, thanks to the Commerce Clause, as applied to marijuana it is.
There are no rules that prevent certain types of legislative activities (e.g. considering and voting for a particular bill), nor can there be. That is, nothing can stop Congress or a legislature from passing a law that is patently unconstitutional (e.g. a law criminalizing criticism of the president). Any such law would simply have no effect, other than to clutter the statue books, since it can't be enforced.