First off, Valve, who develops TF2, Half life, etc. understand splitting an online user base spells death to the community. So they make all their updates mandatory, and best of all, free. That way, the community is never split, the community sticks around knowing their game only gets better and better, and others see the thriving community and buy the game knowing that its a good investment. Valve makes money, gamers are happy.

For the preorder characters, you're right. But multiple preorder bonus shenanigans is another argument entirely. The new guys were just a cash grab. The characters were done and taken out of the game for the sole purpose of "selling" them to us later. See Scattorshot in the credits and the the multiplayer trailer advertising the game. He was obviously implemented and then locked out. Same goes for Onslaught. They should have just been unlockables in the game to flesh out the roster from the get go. Same with Deadend and Zeta Prime.

It should have been a forced download for the characters, but simply not the maps. It would have came with a patch or balance update. The maps would not be downloaded unless you buy the packs. That is also another argument since the maps are already splitting the userbase. It interrupts matchmaking since the game doesn't detect if you have the map until you attempt to join and then kicks you back into the menu again. Vetoing a new map can also bring up another new map, meaning that you are forced to quit, even if no one in the game has one of the maps they can still pop up, interrupting the base game significantly.