I think it's because they may be active, i.e. internally powered, with relatively long life-spans and the ones I've heard of for medical use produce radiated power equivalent to that of a mobile phone for very short periods.
RFID is available in two flavours; active or passive.
Active tags have an on-board embedded power supply, i.e. a battery, whilst passive tags obtain their power from the reader and can therefore be manufactured in a variety of small sizes.
Active tags have a limited lifetime due to the on-board battery, whilst passive tags have an unlimited lifetime.
RFID infrastructure is also something that needs to be carefully assessed such as operating frequency and reader location etc.
RFID is not a new technology having been used in retail for inventory tracking for some time now.
Wal-Mart have been one of the largest RFID proponents with Tesco and M&S also now using it however, it is new within the medical environment.