Spinach actually has a naturally bitter taste due to the prevalent amount of oxalic acid in it which in turn causes this taste along the formation of calculus. Spinach is very high in minerals, vitamins, and nutrients which the body needs to stay healthy. It is also high in potassium and potassium salts like KCI which tend to be on the salty side.
These potassium salts are often used as substitutes but are not considered to be very good ones because of the fact that they have a slightly bitter aftertaste to them. Overall you will find that spinach is much more bitter than it is salty, which is precisely why some people add salt to it.