It's more of a compounded issue. The main reasons people are pissed about this, as far as I can tell from Reddit, are as follows:
1. Putting ads into a game that people already paid AAA price for. This is a relatively new tactic of normalising aggressive monetisation in the AAA space and people are raging like they did when paid DLC for content already on the disk became a thing.
2. They were added AFTER the reviews had gone out and however many people have bought the game, don't need to explain why that is unethical.
3. Combine the previous 2 points with the general feeling of disappointment with the game. It launched with a lot of issues, had more features removed than added (at least ones people cared about) and instead of spending their time correcting those issues they have added advertising to the game. This appears to be the main complaint, as people are taking it as further insult to injury.
I got my copy for free from a mate who hated it, so I haven't lost anything, but if I spent money on it i'd be livid too.