That is a plausible but pessimistic view of the situation. The more optimistic reading is that american workers derive personal pleasure and fulfillment from the work itself. If you are happier working in a fulfilling job than sitting idly on a beach, then it would make sense you wouldn't use all of your vacation days.
Work guilt probably does play a role, as does corporate policies that subtly punish vacation, but I would also bet that some percentage of workers actually enjoy what they do.