it has been argued by many that players "should know the rules" and while the rules for their characters should be very well known, I think players know way too much about what the DM may have up his sleeve, and this is wrong in MY opinion. In the infamous "zombie vs hydra" scenario from a different thread, the "players" are only bent because they know what the hydra's stats are, so if the DM says, nope, sorry, this monster (the hydra) cannot be knocked prone, everyone goes bat crap. Yet if you attack a zombie with poison, and the DM says nope, sorry, poison doesn't work on this monster....the likely response is "oops, I forgot"
In the last game I ran, one of the players was a polearm fighter, pretty well optimized to do what polearm fighters do: slide and knock things prone. And lo he did that to just about every enemy he found. He could slide giants, dragons, demons, whatever. But, put a 4' tall dwarf in front of him and it was one less square of forced movement, a save to be knocked prone, and if he didn't get the full 3 squares of slide on the dwarf because of their racial then he couldn't even attempt to knock them prone.
But, the game was internally consistent. And that made all the difference. In fact, one of the times it didn't seem consistent was when he had trouble sliding a white dragon, which I still hear about to this day. Now, said dragon was a dwarf transformed by a Winter Fey deal. Kept the dwarf racials, you see. But, to the player, it just felt wrong that the dragon was
still a dwarf.
So, as a roundabout answer to your question, the answer is internal consistency. Internal consistency says that, as far as the player knows, and as far as game world physics go, you can slide and knock the dragon prone if your powers say you can. This isn't to say it is
wrong to give dragons themselves the ability to resist forced movement or being knocked prone, per say. However, there is a sort of physics to the world that is expected because of a shared viewpoint between the players and the DM - that being the rules. When they are broken, it can actually detract from the game, lose the immersion, and knock the players into the "game" feeling more than they would be by seeing a dragon knocked prone.
So... expectation. One person might have their immersion hurt by seeing a dragon knocked prone. Another might have their immersion hurt by expecting one outcome but the DM forcing an
unexpected one. In our case, the second is apparently the more powerful determining factor. In yours it is the first.
I hope that has shed some light on the alternate viewpoint for you.