My theory is that a lot of the most rabid complainers about DLC tend to come from folks who have a highly "completionist" approach to gaming, and basically are prone to gaming FOMO.
Like in Batman's case, even if the game itself is perfectly fine and a complete experience worth the $60, the mere knowledge of there being more out there causes some people to get frustrated, because they want to "own it all" and "100% it". In a way, it mirrors the completionist approach to a lot of game design in general ("complete 500 objectives to 100% the game!"), just applied to real life (which is why it feels worse, since money is involved). Since they psychologically feel like they "have" to buy it (only way to 100% it after all!), they lash out at the company for being unfair. This is for the more extreme complainers though...people who just think "meh, that looks like a shitty value, not interested" is one thing, but "this is abusive to consumers! I will never trust them again!" seems a bit over the top imo.
If WB still developed the exact same content on the same timeline, but publicly announced it a month after release, it would be considered more acceptable, even though from the perspective of the game or the development itself, nothing has changed. Everyone would probably praise them for "waiting until the game was finished before announcing DLC", haha.