All the CB's chose to stay their hands. The Fed especially didn't come out smelling like roses...all the dithering and retractions are taking their toll on its credibility. Going forward, they will have a very tough time justifying any decisions they come up with in addition to being ineffective. Decoupling in the making.
By holding rates constant, could be they are expecting a bremain after the vote since nothing would change and any after effects may not be far reaching. A brexit on the other hand will pull the proverbial rug from under their feet and they will be chasing the market/behind the curve for a long time. The ECB and BoJ will have it particularly rough if a brexit comes to fruition.
The main purpose of the stock market is to make fools of as many people as possible.