The passive devices do transmit. They are powered up by the beam of the reader. The simple ones transmit by backscatter. With the std reader they can be read from several meters.
Well, they don't transmit all by themselves.
I guess the gummit could create a super reader and drive through neigborhoods to track creamed corn, but I don't see why they would.
Not exactly what a store/warehouse wants. Hundreds of RFID tags, from "meters" around, responding to a poll going off at the same time, does not add value to the tracking process.
All the ones we used had to be read at less than 1 yard ~ meter. We used them on fork trucks to pick stock out of racks and the last thing we wanted was 40+ pallets responding.
Just make sure you don't pull off the tag on your mattress.