Nobody's arguing with that. The problem is that there was no commitment to avoid querying the data with the purpose of sending it to yourself.
And when Facebook bought WhatsApp, they ceased to be "someone else".
And those include among others that data should be collected for specified purposes, and should only be used for the purposes the person consented to, unless you obtain additional consent.
If I consent to sharing my data for use on site A, and site B buys site A, the fact that they are now owned by the same company is not necessarily relevant unless the permissions collected very explicitly allowed the data collected by site A to be used for purposes related to site B too.
That the sites suddenly have the same corporate owner is no guarantee that the consent collected made it expressly clear to users of site A that they could expect that their data might be shared with site B in the future.
Some sites do collect very broad consent and make very clear to their users that data may be transferred elsewhere, but even if they do this, they also do need to ensure the data is still treated in accordance with EU Data Protection regulations.
I don't think the change of ownership matters. If I say that I won't allow anyone else to query the database, that statement isn't about restricting others from wandering into my offices and pulling up a Python prompt. What I'm really saying is that I commit to not querying the data with the purpose of sending it to others. Maybe that means I commit to not building something to query it for them; maybe it means I commit to not running a mysqld that accepts connections from them; maybe it means I commit to not doing a database dump and sending it, but in all cases, I'm the one not doing a thing.
So, the fact that you "own" the data doesn't mean you have the right to use it how you want - because if you could in fact use it how you want, you could send it to anyone you want. And if you transfer it, e.g., by selling your company, you don't transfer rights that you never had.