I can understand what you mean by it not really being a simulator. It is an experience tho, it gives someone a glimpse of what it was like to operate the Shuttle during the various phases of flight, it has very realistic displays, the means that you manipulate them is also quite real.
Yeah the flights are on rails, it doesnt simulate orbital mechanics at all, but I can understand why they did it this way. It would be very overwhelming for someone unfamiliar to flying in orbit to grasp what needs to be done, and I think it does a good job replicating the way things were done, where you are not just up there flying this thing on your own, you got constant feedback and guidance from Mission Control.
With that being said....there are of course ways for people to actually fly a Shuttle in a realistic simulation of orbital mechanics and have it not be on rails, where if you screw up your burns you end up in the wrong spot and hopeless in ever getting to the ISS. I recommend trying this so you can get the full picture of what it actually took to get the Shuttle to where it needed to go. SSMS does a bit of a disservice in this regard where it makes rendezvous and orbit changes seem very simple and straight forward when in reality, they were not.
Not to mention it still kinda bothers me to see my speed continue to decrease during the post de-orbit burn coast period as I wait for Entry Interface. That right there shows you how it is not simulating gravity at all. But it doesnt bother me enough to not want to play the game. Still a fun experience worth a look for any space fan out there. SSM2 be real or not, the 2007 version still is hours of good fun.