Why is it ethically problematic? There is no implied assumption that I will buy something when I walk into a store. People comparison shop all the time.

And as long as you aren't damaging the books, and as long as the store isn't so crowded that you're discouraging other potential customers, the store looses absolutely nothing by having you there. In fact, they always gain something - there's always the off chance you'll see something and decide you don't want to wait for it to ship.