Federalism is a bitch, consider state versus federal interests:

  The “dual sovereignty” doctrine is not only tied into the existence of two    
  sets of laws often serving different federal-state purposes and the now       
  overruled principle that the Double Jeopardy Clause restricts only the        
  national government and not the states,[55] but it also reflects practical    
  considerations that undesirable consequences could follow an overruling       
  of the doctrine. Thus, a state might preempt federal authority by first       
  prosecuting and providing for a lenient sentence (as compared to the possible 
  federal sentence) or acquitting defendants who had the sympathy of state      
  authorities as against federal law enforcement.[56]                           

  55 Benton v. Maryland, 395 U.S. 784 (1969), extended the clause to the states.

  56 Reaffirmation of the doctrine against double jeopardy claims as to the
  Federal Government and against due process claims as to the states occurred
  in Abbate v. United States, 359 U.S. 187 (1959), and Bartkus v. Illinois,
  359 U.S. 121 (1959), both cases containing extensive discussion and policy
  analyses. The Justice Department follows a policy of generally not duplicating
  a state prosecution brought and carried out in good faith, see Petite
  v. United States, 361 U.S. 529, 531 (1960); Rinaldi v. United States, 434
  U.S. 22 (1977), and several provisions of federal law forbid a federal
  prosecution following a state prosecution. E.g., 18 U.S.C. §§ 659, 660, 1992,
  2117. The Brown Commission recommended a general statute to this effect,
  preserving discretion in federal authorities to proceed upon certification by
  the Attorney General that a United States interest would be unduly harmed if
  there were no federal prosecution. NATIONAL COMMISSION ON REFORM OF FEDERAL
  CRIMINAL LAWS, FINAL REPORT 707 (1971).

[^1]: Constitution: Analysis and Interpretation page 1490 - http://www.gpo.gov/fdsys/pkg/GPO-CONAN-2013/content-detail.h...