For any dive shop to commit to specific sites for a specific day in advance would not only be irresponsible, but it would not be taking the other paying divers into consideration.
Im surprised that your LDS was "surprised" by this. This tells me they are clearly not familiar with how daily boat operations work and the logistics and variables involved.
So unless you are chartering a private boat, it is not a reasonable expectation for you to have them promise specific sites, reservation or not. Depending on who you are checking with, there are anywhere from 5 to 15 other divers in board who also have reservations and are paying for their dives AND most likely who are diving multiple days, not one day. Where they have dive on previous days, their requests and skill level must also be taken into consideration. Sea conditions also change daily so that is another variable in which sites are chosen each day.
However, typically the first dive for most shops is an 80-100ft for the first dive and a shallower dive for the second dive.
I see that you are a new diver. Once you dive some more and experience different places you will begin to understand how it works.
Most of all be safe and make sure to communicate your actual experience with your dive shop of choice do they can do their part to keep you on safe and appropriate dives for your skill level.