You're right that it should not matter. That would be a great world to live in! It's not this one, though. Companies ignore these agreements all the time. Sometimes they're even caught and their wrists get slapped.
More often (I believe) we just never learn the agreements have been broken in the first place.
But it is a rule—almost approaching a law of nature—that companies facing financial distress will begin putting a price tag on private data they've promised never to sell. It's like the cartoon with the starving people in the life raft: they look at your data, and suddenly they don't see a legal agreement to protect it, they see a juicy drumstick.
> After all we wouldn't talk about Dropbox being sold resulting in ransacking of your personal data why is that in the conversation with 23andme?
Well, opinions differ on that one too!