Your point is valid, in that every other motor vehicle that operates on public property has a requirement to be able to be positively identified to a owning entity. But there is no requirement that this be identifiable from great distance.
A drone in the air does create a unique challenge that a transponder would help resolve. But just like any other form of identification, it's an 'honor system' in that I could just put any vessel ID number on my boat, could steal a tag, or transmit a fake transponder ID if I were engaging in illegal activity. A trivial matter if I am advanced enough to be conducting operations with unmanned aircraft.
So I'm not sure that resolving someone's curiosity about my legal activity is a compelling enough reason to require me to attach a broadcasting identification device to my drone, or otherwise identify myself or my aircraft to law enforcement 'just because they should know'