Why is it so difficult to determine when a minor gift, entertainment, or incentive constitutes a bribe?

The biggest difficulty in identifying whether something is given as a bribe is knowing the giver's intent. If it's relatively certain that he or she could gain from this perceived generosity, then a determination of suspicion would be the most logical response. If someone isn't certain of another person's intention, therein lies more of a reason to be more split on the issue.