You forget that the Koranic injunction is 'strike at the necks of the infidels' not 'behead the infidels'. Theo vanGogh's murderer slit his throat, then stabbed him without bothering to try beheading him.
I can tell you this, if they had been beheaded, which the world became familiar with Iraqi jihadi videos, yeah, I might have figured it religious. But murderers stabbing necks is nothing unique in American crime.