
Docking is only legal if performed by a vet.
Boston terriers aren't a breed that's customarily docked, so this shouldn't apply anyway. The breed standard section on Boston terrier tails says: "Set on low; short, fine, tapering, straight or screw; devoid of fringes or coarse hair, never carried above horizontal."
It sounds to me as though their tails, like Bulldogs, are naturally short.
Hope this helps.