I really don't understand why they are limiting ships. I've got access to all 3 at the moment but surely it'd drive sales if they allowed people access to all of the available ships? By all means limit what people can see in the hangar, but when they get into the simulation? Give them a choice of ALL ships.
I agree with what you're saying, it would be better and not unreasonable to let people access all of those ships for the DFM, in the same way they allowed the 2 TNGS ships to be in our hangar without us owning them.
The issue with what you're saying though, is that this doesnt fit their story, their lore or whatever. The simulation is being run on the ships computer, with that ships load-out. So in order to say 'let us play/select any ship' you'd have to break that version of events, that piece of the games reality.
It wouldnt be soul destroying to the game if they broke the 'immersion' of the ship computer running the simulation, and simply acknowledged this is all a computer game and not the finished version, so lets not bog ourselves down with finished mechanics, and just let people play... but there is some logic to it, even if it shouldnt be important at this stage.
I also think you'll get quite a few people who bought their Aurora package upgrading to or buying a 300i after the positive feedback from others, and that equals sales, so theres financial reasons to the restrictions too.
AFAIK, the subscribers have a pass which lets them try others, but even this is time limited, like a 24h period of access (per month, i'd assume). So thats considered a perk for subs, and while im not one, they certainly need something IMO (not that i think this makes it worth it, personally).