No, it wasn't. The Proclamation Line was a quite reasonable and perfectly LEGAL boundary established to keep the British colonists and the Indians in the western backcountry separated until such time as the Royal Government Board of Trade could negotiate a final land settlement with the Indian tribes and set up a permanent boundary.
Remember that the colonies were still under British law in 1763 and the colonists considered themselves as British---they weren't angry at that point, but were relieved that the British Army had defeated the French and their Indian allies.
In the aftermath of that war, there was a lot of uncertainty in the "middle ground" between colonial America and Indian country. So the British struggled to come up with a fair land policy that would satisfy both sides. Believe it or not, King George was trying to do the right thing. He had to since the Indians, unlike the French, really had not been defeated yet---that would not happen until the Rev War and the later Indian wars of the 1790s through the War of 1812.
So, the Proclamation Line was hardly arbitrary but quite deliberately surveyed and laid out along the Appalachian Mountains and the rivers and waterways which served as natural boundaries. After Pontiac's rebellion, which scared the British, the Indians honored their agreement with the British, but colonists like Washington and his land speculator cronies did not.
The primary sources don't lie---it's all laid out in Washington's correspondence and the Board of Trade Records in the UK Public Record Office if you care enough about this subject to do some research.