The problem is with privacy and privacy laws. If you read the laws about privacy (like http://www.ico.org.uk/for_organisations/privacy_and_electron...) you are going to see that they are targeting cookie-like tracking systems.

It's not a technical problem at all. Tracking is as easy to solve as to put a UID to each device (problem solved).

e.g. Apple some time ago forbid developer to use such an unique id (http://www.tuaw.com/2011/08/19/ios-5-deprecates-udid-as-iden...) probably on privacy laws grounds.