In the current edition of the rules for this great series (as published in Monmouth and dated 2007), rifle units can only fire at adjacent enemy units. It would appear that the rule may have been changed somewhere along the line.
"Rifle unit: A light infantry or infantry unit which is armed with rifles. Rifles permitted units to fire over greater distances than units solely armed with muskets. Rifle units are designated by a white ‘R’ in a black circle on the counter."
What is the benefit of having rifle units if their range is limited to 1 hex? I think the correct interpretation should be that the Rifle/Artillery Fire 'To Hit' Table should be construed as just that. Rifles have a greater range than muskets and can target enemies up to 4 hexes away (if strength > 1)
I think perhaps the adjacent bit in section 10.2 is an oversight that hasn't been corrected yet.
The benefit is that Rifle units are allowed to fire in the Rifle Fire phase (they still must be adjacent to target) as well as attack in Close Combat phase (but they are not required to attack in Close Combat phase, unlike musket-armed units).