I think the word "problem" is a bit of a stretch. It may not have as much actual value -- dollar cost of books read in a month/$9.99 -- as the perceived value -- unlimited. However, a successful product is about perceived value, not necessarily actual value. As the parent comment mentions, that convenience of looking something up without worrying about costs or alternate searching, and the freedom to not have to worry about the price of individual books, or having a shared kindle unlimited for a whole family with kids you want to encourage to read, are all worth something to a customer that can't be assessed in dollars. And of course, it's up to the customer to evaluate if they actually have the needs fulfilled by the product.

It's definitely a brilliant business move on Amazon's part.