I don't think anyone who understands the issue is complaining about them monetising. People know it costs a ton to maintain the infrastructure. That's not the point.

• people don't mind TPAs are billed, but the pricing is a thinly veiled attempt at pretending that they don't want to completely eliminate TPAs. People would have been fine if the pricing is reasonable. They don't like it when you try to pull a fast one over them.
• their own application is bad. There's no way around it. From accessibility to mod tools.
• they have promised to focus on the above points for years now, and there's still almost no improvement. People don't trust them anymore.
• spez's recent comments against the Apollo dev are blatant lies and it only fuelled the outrage. Unfortunately for him, the Apollo dev legally recorded the conversation.
• they do own the data, but it's still the users who create them. If you treat the people who create content for you like trash, expect backlash. This happens on many private companies, not only reddit.

There's more, but I'm out right now so I can't focus much. Basically, if your content is from the users, you should take care of the users and people running your site.