They're RFID, so they're not putting out any signals until activated by an RFID reader. They're no more dangerous than a credit card with an RFID chip, or an employers' ID badge with an RFID chip to unlock doors that one might carry with them on a plane when flying for business.

And if you're worried about toll booths on the ground reading it and charging your account - they're not going to read it when it's 30 feet away, they're certainly not going to read it when it's 30,000 feet away. If your plane gets close enough to a toll booth to read your EZPass, then you've got bigger problems than a few bucks being charged to your EZPass account