While there are thousands of federal criminal laws, not every violent or heinous act qualifies as a federal crime. What almost all of the federal crimes have in common is the requirement that the crime in question impact interstate or foreign commerce.
The Supreme Court made it clear in its landmark decision in United States v. Lopez that the commerce connection was a real limit on the expansion of federal criminal law. It held that Congress overreached in passing the Gun-Free School Zones Act because the possession of a gun near a school zone was a local matter. The federal government failed to show that having guns in a school had a "substantial impact" on interstate commerce, the Court declared.
The statute under which Tsarnaev is charged on the first count—use of weapons of mass destruction—was passed into law as part of the Violent Crime Control and Law Enforcement Act of 1994, which was once best known for banning assault weapons. When it was passed it was the largest federal criminal bill ever. It included federal funding for new police officers and prisons, and a variety of new crimes relating to gangs, sex crimes, immigration, and terrorism.
The WMD section of this law is very explicit about the interstate requirements. A person can only be guilty of violating this section if he travels out of state or out of the country as part of his plot, or uses the mail, or "the offense, or the results of the offense, affect interstate or foreign commerce, or, in the case of a threat, attempt, or conspiracy, would have affected interstate or foreign commerce."
There no evidence that Tsarnaev, a Massachusetts resident, did much traveling to carry out the attack in Boston. Instead, the prosecution appears to be pinning the charges on the idea that the attack or its results "affect interstate or foreign commerce." That is to say, the government says this is a federal case because of its commercial impact.
The affidavit of the FBI agent that accompanies the criminal complaint is very explicit about this.
The Marathon has a substantial impact on interstate and foreign commerce. For example, based on publicly available information, I believe that the runners and their families -- including those who travel to the Boston area from other states and countries -- typically spend tens of millions of dollars each year at local area hotels, restaurants and shops, in the days before, during, and after the Marathon. In addition, a number of the restaurants and stores in the area near the finish line have special events for spectators.
Back in 1996, lawyers for Timothy McVeigh, the guy who blew up the federal building in Oklahoma City, argued that WMD law should be declared unconstitutional on basis of Lopez. If Congress can't ban guns near schools on the vague grounds that education impacts commerce, the constitution should also limit its ability to criminalize weapons of mass destruction, the lawyers argued.
The federal court swatted this argument aside by saying that the impact on interstate commerce of a mass bombing is "both obvious and substantial." The Oklahoma attack, the court said, produces consequences ranging far beyond state or local interests."
"If the evidence at trial proves the factual allegations of the indictment, the impact on interstate commerce is both obvious and substantial. The use of a truck bomb of sufficient explosive power to destroy an office building, killing and injuring hundreds of its occupants, has a substantial effect on interstate commerce," the court declared.
Nonetheless, the court did say that the question of impact on interstate was a factual matter that the jury should decide. That means that the prosecution in the Boston attack will have to show that there was an effect on interstate commerce from the attack.
The other charge against Tsarnaev is the deadly destruction of property charge. Once again federalism and the limits of Congressional power to pass criminal laws play a role. Under the law, federal criminal liability applies only if the property that is destroyed is used in interstate commerce or in an activity affecting interstate commerce.
In 1985, the Supreme Court held in Russell v. United States that a guy who tried to set fire to his own building (probably for insurance purposes) could be guilty of violating the law because he rented out apartments in the building. The legislative history, the Court held, "suggests that Congress at least intended to protect all business property, as well as some additional property that might not fit that description, but perhaps not every private home."
The presence of business in the area that was attacked, including hotels, is almost certainly enough to trigger federal criminal liability in Tsarnaev's case.
In short, although most people do not principally think of the Boston Marathon attack in terms of its business impact, this is what gives federal prosecutors the ability to charge Tsarnaev federally. And to seek the death penalty for a crime committed in a state with no capital punishment.