Why do companies do this? I don't care that "many" of them do, but why is it done? The OP placed her order for specific items and Macy's knows what her order is, so what purpose does it serve? What is Macy's protecting itself from? It's not like the OP can order more items and not pay for them.
I have always known that the gas station I use authorizes for $75 when a swipe my card and then the additional falls off in a day or two, but retail stores holding up to 3x the purchase amount seems over the top.