The Federal Aviation Administration ordered airlines to remove the oxygen tanks located in lavatories in 6000 aircraft across the United States. The FAA determined that someone could use the oxygen tanks to start a fire or to create a bomb. The Air Worthiness Directive 2011-04-09 stated that airlines had to comply over a 21-day period, ending on March 4th. The entire process was kept a secret to the general public, since the FAA did not want someone to use the information for evil doing.
This means that if cabin pressure is lost mid-flight, there would be no oxygen mask for a passenger in the bathroom. This has some people very upset (and afraid), but really flight attendants are trained to take care of you and although some people think depressurization happens all the time it is a very rare situation — especially when you think about how many flight hours are completed in the US on a daily basis. I would rather take the risk of being in the bathroom during decompression versus someone using the oxygen container for something bad.
So why have the bathroom oxygen tanks been removed, but not the others in the main cabin? I suspect the ones in the lavatories were targeted since passengers have a pretty high level of privacy and could barricade themselves in. The airlines and aircraft manufactures are working on a better solution (I am thinking an alarm to access the oxygen tank?), but there is no word when oxygen will be re-activated in the lavatories.