Darryl, I understand your point and you are not incorrect in being unhappy about it.
However, there are many businesses today that charge different prices to different customers at different times. The airline industry, although not a perfect analogy, uses this method of pricing constantly depending on the time of day, time until flight, how full the plane is etc. Even different travel agencies get dished different prices. Even supermarkets send coupons to selected deomgraphic groups, which, in effect, is discriminatory pricing.
I think that if the line on the guest web page is different than what you get after signing in, this can be seen as deceptive. The cookie method would appear to be more fair on the surface. At least the line won't change on you as you sign in.
The main problem I have with it, and why I avoid these books, is that the line services that display these types of books are always incorrect. I see a line and go to the site only to see that it's not what I get. It makes it very inconvenient to line shop. Any book that contributes to this inconvenience runs the risk of customers just not bothering to check with them anymore.