It is my food to deal with no one else s.
Well...not really. It depends greatly on the culture of your realm, amongst other things. For starters, the game specifically tells the ruler that all the lands belong to him, so technically it's his food. And there are plenty of realms that expect nobles to part with their food for free. And any region lord that tells his duke, banker, judge or ruler that, "This food is mine, I'll charge whatever I want for it so to hell with you" probably isn't going to be in the realm long.
Edit: You've also ignored the whole "It's to stop exploits" reason. You seem to think having any cap at all will stop the exploit, whereas it's more likely that the higher the cap, the more gold exploiters can gain. Cap = 50 gold, not much profit from the exploit. Cap = 500 gold, as you suggested, lots of profit for exploiters.
