Possession/sale of marijuana is illegal under federal law, but more states are legalizing marijuana (for medical or recreational use) every year. US Attorney General Jeff Sessions objects to marijuana legalization and has vowed to aggressively enforce the federal prohibition. 

Question 1: Is it fair that the federal government may criminally prosecute people who are acting legally under state law but criminally under federal law? Why? Why not?

Question 2:  If it is NOT fair, why does it happen?

Question 3:  Can the federal government regulate anything? What are the limits?