Actually, it is an excellent marketing tool. It is all about being inticed to book.
Cruise lines do it as well; let you go through the entire search process and then they say it is too early to book. They are trying to entice you to book an earlier trip within the booking window.
I am waiting to book my cruise to Bermuda for October 2014; cruises you can book up to 18 months in advance. I know I cannot book it yet, but a girl can dream......it allows me to go through the entire process of choosing the departure port, number of days, etc. and then gives me alternative dates that are within the 18 month booking window.