Found an interesting tech article on the J&P website regarding motorcycle oil coolers and thermostats:
"Whatever type of oil cooler you choose, make sure it's installed in conjunction with a thermostat, designed to open at no less than 180°. This is where the "system" in oil cooling system comes in. Many folks have actually damaged their engine by installing an oil cooler straight up without a thermostat. The result is oil never reaches operating temperature and thus fails to scavenge contaminants from the engine. If the engine is using a heavy grade straight weight oil, there may be additional damage from oil starvation at critical lubrication points. If you can't for some reason install a thermostat, at least put a cover over the cooler in temperatures below 80°. It's not as effective (or easy) as a thermostat, but it's better than nothing!"
I would suspect that this is why Harley coolers include a built in thermostat.
Makes sense to me, especially if you ride your motorcycle in cold weather.