I guess someone in accounting figured out that you need money to continue paying your employees.
Discussion
It's not charging for the API that I have a problem with, like you said these things cost money to run/develop. It's allowing outside companies to buy equity in OpenAI by creating a for-profit company under the non-profit parent company. That move basically made the "independent non-profit" label of the parent company meaningless and frankly, bullshit.
It was inevitable imo and I’m sure they knew that. It makes you wonder why they pretended to be “open” at first.