There's an even more compelling reason why Apple's waiting a while before announcing the tracking tags: crowdsourcing.
At WWDC this year, Apple announced that all iOS devices would begin reporting nearby devices' locations back to Apple. U1 probably only works at relatively close range, so they want to wait until their "crowdsourcing network" includes enough devices for long-range tracking to work. (IIRC they aren't going to let users prevent their devices from contributing to the network, so it's just a matter of waiting until everyone has the latest iOS update.)
At WWDC this year, Apple announced that all iOS devices would begin reporting nearby devices' locations back to Apple. U1 probably only works at relatively close range, so they want to wait until their "crowdsourcing network" includes enough devices for long-range tracking to work. (IIRC they aren't going to let users prevent their devices from contributing to the network, so it's just a matter of waiting until everyone has the latest iOS update.)