On Yamaha skis they put it there as a safety feature (according to their
PR folks). They want you to have to take your hand off the throttle to operate the reverse, because of the potential bad results that could happen if you engaged the reverse at speed (either you get pitched off or you break the reverse bucket).
I figure you are asking about Yamaha, since the reverse lever is on the port side on newer Kaw, and SeaDoo skis. You can also find owners of those skis that have had the misfortune of shearing off the reverse bucket when they accidentally activated it at speed. You could probably do the same on a Yamaha, but you would have to work a little harder.