Then why would a paladin be oathbound but a cleric of the same god is not?
Why can't nuns perform marriages? Why can't most mayors carry guns and arrest people? Different oaths for different positions with different responsibilities and consequences.
The paladin's oath is directly to his god; the cleric's is to his faith's heirarchy. The paladin's duty is to be his god's sword arm; the cleric's is to minister to the flock in times of peace, war, sickness, health, happiness, sorrow, etc. This also includes power over paladins- their ability to cast Attonement acts as a check on the worldly power of the holy warriors. It keeps them in a position of subservience to the faith (humility IS a virtue...and a rare one at that).
A cleric's oath may be minor in power compared to the pally's: a god has need for many priests to do all those things for all those people, so the penalties for violating the oath may not be so great. But make no mistake- if he makes the wrong kind of error, he will be struck down. (Scour the various divinities through the editions and you'll find those who kill those priests who dare convert to other religions, for example.)
Returning to Arthur, whether or not he was a paladin, his oath as king has a different set of responsibilities that supersede all others. If, as some assert, his legends were based in gaelic legends, his sins would affect not just him, but the entire kingdom. And he couldn't raise the dead either.