Actually they don't have to, they could be designed same as a car. The splines that engage the wheel would still have to be lubed, but the splines on the shaft itself could be lubed with the gear oil in the final drive on the rear (as they are on a GL1200) and by engine oil on the front. All that would be required is a slightly different design, and oil seals front and rear (like cars have)
If you are going to have a design like the Vulcan, at least you could put grease fittings on it. There could be a grease fitting on the rear coupling and on the U joint, with caps on the swing arm you could remove to get to them. The fittings would be on the shaft itself, and would spin with it. Most replacement U joints for cars and trucks have grease fittings on them.
All of our construction and landscape equipment has lots of grease fittings, one backhoe has 40 of them, and they are supposed to be lubed after every 8 hours of operation.
My Yamaha XT225 dual sport, which was made unchanged from '92 through '07, has 5 grease fittings on the rear suspension. When it was replaced by the XT250 in '08, the grease fittings disappeared, but I'll bet the lubrication intervals are still the same.
I lost my Vulcan owners manual some time ago, but I'll bet it says that the spline lube should be done by a Kawasaki dealer.
BTW, that XS1100 Special was one of my favorite bikes. It was a cruiser, it did not look like a Harley, it was comfortable, it handled very well, and it was FAST. Yet the inline four was very primitive by todays standards.
EDIT: Just look at this and tell me that is not a beautiful motorcycle.