The 10% are the good-for-nothing that will be stirring trouble as long as there's no watchmen to kick their ass.
When your taxes are set to none, 10% of the citizens are still marching up and down Main Street, protesting the absence of government services and places of worship.
I.e. no matter how low your taxes, you still get 10% unrest. You need a bell tower/tower of dominion/something to reduce it to zero.
The tooltip is unclear, and the unrest is present in your native cities, too.
