What's key here is consent. (the real kind, not the EULA kind)
If you knowingly opt into being identified (like ticking an unticked "remember me" box that clearly explains the precise purpose of the identification) then it is okay.
If some asshat decides to do it without your informed, clear, and uncompelled consent, throw the fucker in the slammer.
Remember Me On This Device: Not today Satan!
Use code LINUS for immediate jail-time.
Find My iPhone — You mean find my digital privacy regulator.
I think the only way you're gonna swing it legally is by intention.