They usually are, but if you've got a receiving coil then you can put in a lot more energy before you begin heating up the primary coil. This is because with the other coil there it'll take in energy that would otherwise be turned into heat in the primary coil if you weren't draining it off. This means that if you can detect the presence of the device (usually by a change in the resonant frequency of your own coil I think is how they do it) then you know that it's safer to dump more energy into since it won't all be turned into heat.