The loads on the clutch are dependent on the torque from the engine, not the weight of the vehicle.
There's two torques, one from the engine and one from the available traction. The clutch sits in the middle. So I think the loads from inertia and shock on standing starts are an issue. I was using the word "load" in a more general way then you are though, I was thinking the "heat load" from standing starts would be much higher, the extra time and throttle used to get a car weighing about twice as much as the bike up to a point where your not using the clutch anymore would be much higher. I don't think we disagree though...
I have a very nice F3 race clutch for the race car, but it is not considered adequate for standing starts. If I do hillclimbs or run with a club that does standing starts, I need to run a stock clutch.
True, the second torque is just a reactive force from the wheels. But, to that effect..
The bike has maybe 1/5th of the car's contact patch overall (so effectively 1/10th of the total amount available for the driven wheel), but it applies a higher load to that patch (lower coefficicient of friction, higher overall grip due to Ff=uFn) than the car, up to 4x higher loading per square inch than a normal car, so the gap isn't quite as wide it may appear. Furthermore, the bike's tire deforms proportionally more on acceleration, further increasing contact patch size in the first split second of a launch. If neither of those happened, then you'd NEVER see a liter bike put down sub-3 second 0-60 times because of lack of traction, but most manage it just fine nowadays, on road tires. A hayabusa will knock down 1.5 second times, even. Still plenty of grip (i.e. not spinning the wheel constantly), and right on par with a BEC doing it in ~4 sec (simple f=ma, double the weight, ergo half the acceleration), which tells me that its clutch is dealing with very similar forces, except for twice as long.
Plus you have to factor in that a diff in a BEC effectively lowers the road friction torque to the clutch to a degree not seen on a bike, for example a 'busa is 17/40, or effectively a 2.35 ratio, much lower than you'd be able to find for a BEC, ergo the torque seen at the clutch from the road is lowered with a higher ratio diff.
Also, bikes have larger rolling diameters (17" w. 190/50/55's, for example), which is also taller than you'd see on a BEC, therefore less torque through the driveline in the first place. There's a lot that plays into this besides just contact patches.
Heat load is higher, agreed, though the multi-plate and wet-clutch designs nearly all motorcycles (Duc's excepted) use go fairly far in helping that. Though once again, 4 seconds of load at a time vs 6 (bike vs bec, quick but not max acceleration) still won't exceed the stock clutch rating by any stretch. Perhaps 'identical' is a bit of a strong word, but I'd be shocked to find a difference of over 10% or 15% in wear between the two, considering how little use a clutch on a motorcycle engine actually sees. A biker could use a clutch harder and get less life out of it than someone in a BEC who knows how to drive it, and treats it with respect. It's more down to technique than technology, IMO.
Ok, threadjack done, sorry.