If one plans to travel from A to B and then, later, along the same route, from B to A, and one wishes to purchase a ticket for both components of the trip, one will, if one is a speaker of British English, ask for a return ticket. If one is a speaker of American English, one will, under the same circumstances, ask for a round-trip ticket. While the British phrasing is not perfect (as has been discussed elsewhere on this site), the American one is downright puzzling, given that there is usually nothing round about round trips; the route traversed on a round trip is, in most cases, much closer to a straight line than to a circle.
It wouldn't be puzzling if the term round trip were used for a trip such as A–B–C–D–E–A without backtracking on any part of the route, but the term is not reserved for such cases; its most frequent use is for plain A–B–A trips. (Some travel websites, in fact, use round trip only for trips of the A–B–A kind, and characterise those of the A–B–C–D–E–A as 'multi-city'.)
So, the question is: how and why did round start being used for this purpose in American English?
(I am not asking when it started being used for this purpose, except in so far as it may throw light on why that word was chosen.)