I know I'm not an admin, but I was there... so here's my $.02
1. First it was just a temp ban which is probably over already or soon.
2. You had been warned
3. I died in time to see exactly what you did when you hopped down from the roof, and what you said is what happened.
That being said... I've held that roof myself as an engie after the third point has been taken. If the sentry isn't able to see the spawn door and you can safely stand outside of the spawn door, it shouldn't be considered spawn camping. Especially if the defense was there before the spawn moved. The definition of spawn camping is somewhat obscure, as it varies between people and moreso during map progression. You can legitimately shoot into spawn when a round starts if you're red, but as the map progresses that may be considered camping.
But, I go back to point #2. You had been warned. Whether you agree or not, an admin holds the trump and their decisions stand and have to be respected. It is their server. At least you got a warning and a reason.