The actual cost is shrinkage from general human accounting mistakes and all the extra time it takes to manage.

I worked at the gym in college and we sold like one item a day and it was still a whole bunch of work and pain to keep up on the cash counts correct.

I definitely believe that all businesses should take cash as much as is reasonable, but logistically it is understandable why some choose not to

You shouldn't do that anyway; also, you can't skim a credit card I'm not using/carrying. There are crime arguments on both sides.