The reason is that if one placed the meat in a vessel with water but it had no holes, the meat would then absorb the blood and salt that is on it.
 It seems that the Rema is not arguing with the Mechaber, and is simply ruling that a spraying or wiping of the meat followed by two intensive rinsings would be acceptable. Regarding the Rema’s second view of requiring three rinsings - it is because the large quantity of water used will nullify the intensity of the tzir and an initial wiping/spraying would not be needed.
 Post facto, if one only rinsed off the meat once and then cooked it, it is acceptable,
 This is based on the Gemara in Masechet Chullin, daf 33, that says if one slaughtered an animal and blood did not come out, then one may eat the meat with hands that are tamay. Thisis because the meat is not susceptible to tuma due to the fact that blood did not come out. Tosafos asks: Shouldn't the meat become susceptible to tuma when one rinsed it out in the water'? The two answers given are the a) the meat was to be roasted or D) it was rinsed in fruit juice. Therefore it seems that fruit juice would be permissible even for the preliminary washing, in contradiction to the Rema that holds it would be permissible only for the washing after salting.