That may not be true. Using HCl (hydrochloric or muriatic acid) would cause bubbling in a lot of materials besides calcium, and that acid is less pleasant to work with than a solution of EDTA (which is cheap). Some dip tubes, as I stated earlier, are composed of polymers stabalized with cationic species to help stabalize the resin. This is of particular interest to me since I am an analytical chemist, and my girlfriend works designing resins (although we do not work at the same place). The EDTA titration is very straight forward, inexpensive, and more sensitive than a qualitative Ca/Mg scheme. The EDTA method is also the method approved by the EPA, since it is precise down to the parts per million range.
The Friendly Chemist