This isn't about people typing furiously on internet forums. This is about business risk, as weighed up by people whose job it is to care. Acquisitions take time and negotiation. Just because a startup is talking to Twitter, doesn't mean the acquisition will definitely happen. But if customers get wind of it, then they may start looking to move elsewhere, anyway, instead of waking up to find services they rely on have been shutdown without warning. If you're the customer of a startup Twitter is negotiating to acquire, then you're negligent not to reduce your reliance on them. And if you're the CEO of that startup, then you have to consider the risk of customer flight (and your own comfort with the thought of screwing them over) vs. the likelihood of the acquisition happening.
Why would anyone else?
I think potential for reputation damage, on an activity they don't do very often, and where they will justify it, will be weighed up against the financial benefit now, and disregarded.
I think it's possible to overstate the relevance and importance of people typing furiously on internet forums about things like this. Twitter is what it is - most people aren't using APIs, third party clients etc and will never notice this.