Hardware hacker learns his E-ZPass is being read all over NYC, not just at toll booths

Over at Forbes, Kashmir Hill has an item about an electronics tinkerer who goes by the nom de net Puking Monkey. Stranger than his handle, his discovery: the RFID-enabled EZ Pass (automatic toll paying system) affixed to his car was being read all over New York City, as he drove around. Not just at toll booths. For what purpose? And, is this a violation of reasonable privacy expectations, in the most heavily surveilled city in the US?