This is legal vs rude. It certainly is legal and was in the terms of service for them to use the data in any way they see fit. But, also it’s rude to bait and switch from being a message board to being an AI data source company. Users we led to believe they were entering into an agreement with one type of company and are now in an agreement with a totally different one.
You can smugly tell people they shouldn’t have made that decision 15 years ago when they started, but a little empathy is also cool.
Additionally: When you owe your entire existence and value to user goodwill it might not be a great idea to be rude to them.