Because if you had the choice to turn Flash on web developers would keep using Flash so us that really don't want to use it would have much less HTML5 content available.
What I don't understand is the rest of the world is using it, it's not going anywhere. Windows, OSx, Android, etc etc all have no issues with using it. If Steve Jobs really wanted to do his part to eradicate Flash he would ban it on OSx and all of the devices Apple sells. Web developers will continue to use Flash, I haven't seen many if any of the sites I visit switch from Flash.
I just want the internet to work, no more, no less.