I can understand the thought behind it. We tap phones, we intercept packages, we follow people around. As long as there is a warrant, there is nothing wrong with that. But now you have this pretty much unbreakable kind of communication that goes around the world that you can't intercept, while it would be very valuable in some cases. Personally I find the whole demand from some for absolute privacy online a bit overblown.
Problem is, there is also nothing stopping a terrorist group from coding its own app and slapping encryption on it. So how valuable is it to demand Whatsapp and others to built in backdoors for the government.
There's also the underlying issue that creating this vulnerability for police usage does not mean it will actually
stay limited to police usage. The medium doesn't help either: tapping phones, intercepting packages, stalking - these all require some degree of physical access to the person and/or materials. Digital messaging, and the interception thereof, throws that on its head entirely; you can be on another continent, having never been in the same country as the target, but still potentially get their info. That's not just for police and terrorists, that can be for hackers wanting blackmail, and the knowledge that a backdoor absolutely exists would be quite encouraging to such criminals. Fine, you feel personal privacy does not fully trump the security needs of society - that's a pretty reasonable stance and the one people always touch on. What they're far less willing to acknowledge is that creating such vulnerabilities for the police will make them potentially accessible by someone
other than the police.
To put it in terms of analogy, if it was known that the police had access to an array of skeleton keys, provided to them by every lock manufacturer, and each key would open every lock made by those manufacturers, the response by criminals wouldn't even be to try to steal those keys; it would be to experiment, knowing there was at least one particular combination that would open all locks of a given manufacturer.