Why the crackdown on Seattle marijuana dispensaries?

The federal Drug Enforcement Administration has sent shut-down-or-else letters to 11 Seattle-area marijuana dispensaries. These are medicine shops, which the federal government supposedly tolerates, not the “Initiative 502 stores” for all over-21s. Those won’t open until next year, if U.S. Attorney General Eric Holder decides to allow them to open. Medical dispensaries are supposed to be tolerated.