Why BlueTooth is activated after updating the iOS? I noticed something while updating the iOS. After finishing the updating, I always find that BlueTooth is on and activated.  

Any idea why Apple is doing that?
Apple turns on Bluetooth because of iBeacon. Also, as previously mentioned by @Andrew Larsson, AirDrop is also a major reason. See here for more details: https://www.yahoo.com/tech/this-is-why-apple-enables-bluetooth-every-time-you-80079149965.html

A quick Google search reveals many interesting results: https://www.google.com/#q=why+is+bluetooth+enabled+after+updating