They want you to call because they most likely have several questions and further questions depend on answers to the first questions. It's cheaper for them to just handle it all in a couple minute phone call rather than sending 5 or 6 email replies to get to the bottom of the issue. They want you to call so it can be at your convenience.
reply to A non OP needs to rember a few things about retail:
1) not all employees are trained as cashiers 2) you don't just open a lane. You need a cash drawer that has been counted, both before and after. 3) That one time was unusual. Stores don't staff for unusual. 4) The manager has liitle control over how many employees are in. The store is allocated a staffing budget by corporate. That is a weekly amount either $$$ or hours. The manager then needs to staff most efficiently whithin that budget. 5) That budget is based on store performance. You throwing your tantrum and leaving will not make the store staff more off hours. Less sales mean less hours.