Hindsight is 20/20, and based on that I personally would say WhatsApp was not ethical even when it was a standalone company. If it had been so, it wouldn't have allowed Facebook to acquire it in the first place! I realize this sounds harsh and probably implies bad motives on the owners, but whatever WhatsApp wrote in its blogs pre-acquisition were all principles that could be sold at the right price. That's exactly what it has come to now that we know how it's being used.
I would also consider it insulting to human intelligence if the WhatsApp founders had truly believed that they would be able to continue with their stated privacy stance after being bought by Facebook, of all companies (what they wrote on the blog at that time can be seen as public posturing).
Cash pressure + lots of valuable assets liquidised legally that just require a change to ToS = said assets for sale + temporary bad PR.
If my company is profitable (or has revenue growth and is likely to become profitable) and a scummy company wants to buy it, I'm going to say no.
Hard as it may be for some of the HN/VC crowd to believe, but there are people out there who start companies with their own money, because they believe in something. At no point does "find some fat megacorp to buy us our" enter the game plan.