I don't necessarily blame the developer for selling: I understand that some offers are difficult to refuse. But I absolutely do blame him for being dishonest to his users and contributors.
No one was told about this. People only found out about the sale by chance, because someone noticed that the Play Store listing details were changed and made a post on Reddit.
When confronted on GitHub, the developer gave evasive answers, citing vague and unrelated issues, such as "the quality of the Android ecosystem dropping".
I assume a lot of users bought these apps with the expectation that they were not infested with ads, data mining, dark patterns, etc. Most people have automatic updates enabled, and they will get all of the above shoved into their face before they can prevent it.
The value of these acquisitions is determined almost entirely by the userbase. The developer was only able to get this deal because of his users. At the very least, they deserved to be treated with some basic amount of respect.