Not that much of a stretch. Since a state may refuse to arrest someone who committed a crime in a different state if they are sympathetic to said crime (especially a concern when USA was split between slavery and anti-slavery states), the feds can have jurisdiction in these cases, since otherwise it may involve a dispute between states. It makes at least some sense…

If you want to see a real stretch (aka completely made up bullshit), look up how feds justify growing marihuana for your own consumption (not sale) somehow involves interstate commerce.