How to prevent an iPad from grabbing Apple ID for "Itunes and App Store" when plugged into a Mac?
I used to get this issue as well and what I did was to prevent iTunes from auto syncing when I connect a device, this option is found in the iTunes Preferences/Devices and tick the checkbox as shown in the screenshot



It's also worth going into the user preferences then login items and remove "iTunes helper" which should stop it from launching iTunes anytime you connect a device.