I never knew it was a form of pica...that's new information for me. There may be any number of reasons why someone chew on a cup of ice: the crunch; moisture due to dry mouth, (some people take medicine or have medical treatments in which their saliva glands have been affected); soothing nausea, (I had vertigo with nausea very badly and this helped immensely); giving up one habit and substituting this as a tool to stay free of the habit, (i.e. smoking).
And, some people enjoy chewing ice. Hope this gives you info to 'chew on'. :)