Unless their margins on coffee are high enough to subsidize a book inventory. At which point, they could basically be a library without the lending.

Not ethically problematic at all, economically problematic for Borders, certainly. As another poster pointed out, Borders offers the same products as online bookstores except with a substantial markup. Markups need to be justified with some sort of value-add, what's the value-add for Borders? Quick access to books? That's a dubious advantage in the age of Amazon Prime and e-books, especially when weighed against the disadvantages of physical bookstores such as time wasted waiting in line, time wasted making a trip to the store to pick up a specific book that it turns out they don't have in stock, etc.

Borders is a retail store heavy company, and they haven't made any significant changes in their store in response to competition from online stores. That's a huge red-flag and is nobody's fault but theirs.

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.