Low Batteries, Battery Tenders, Headlights On - Why not a Relay?

A common problem for bikes that are a year of so old is a battery that is OK for everything but starting. Even happens to new bikes!

Turn the key on, the headlights come on, drawing maybe 120 watts, and the starter turns the engine over but there's not really enough power left to fire the ignition . . . . but for the "constant on" headlights.  

You could blame the small light batteries that bikes now carry but I'd rather keep them and have a relay to cut out the headlights when in start mode.

Is there a technical reason (or maybe a regulatory/legal reason) why our bikes lack a relay switch that cuts the headlights whenever the starter button is pushed?

I don't know, are there any models that do have such a relay?  

Actually, many bikes won't turn on the headlight until the engine starts for exactly this reason (although if you start the engine and use the kill switch, the headlight will remain on). A relay is probably the logical method for this interruption.

The Honda CBR1000RR, however, does seem to struggle to turn over the starter motor, even in brand new condition, and it's something we've commented on in our tests. We've noticed the 2012 model doesn't seem to struggle quite as much as the earlier models.  

I have the same problem with my '86 GSXR 1100 and with my wife's GZ250.

The GSXR's battery isn't small, just old, and those twin endurance style headlights suck about 120 watts.  

The GZ's battery is tiny and it could barely turn over that single fast enough to start.  I added a second larger battery under the tool kit/above the swing arm and now it spins up and starts easily.

What I don't understand is that all three of these bikes already have a relay to handle the headlights' load anyway, so why didn't the manufacturer switch the light circuit off the starter motor circuit, in the first place?