Cruisers get rusty frames too. 60 series probably has the worst reputation for frame rust among the LC models in US. Most of the problem with them is the from the rear axle back, all of which is C channel.
My Canadian 70 series was fully boxed as I recall. It had horrible frame rust. I enjoyed it for a few years before selling to an enthusiast who claimed he'd "fix it". Not sure how that ended. Short of a frame swap or some serious fabrication I don't see how it'd be possible.
80's can rust out there too, but seems less vulnerable than earlier series.