I would like to point out that with a lot of smartphones these days, there isn't a button you can push that will simply pop off the back casing. Anymore, you NEED pry tools to remove these things, because the mechanism holding them there is a simple hook that clicks into place. It's all a solid piece, but you can normally remove back casings without doing damage to the phone. The manufacturers will typically have an area on the phone where you can pry from without causing damage, and if you do it slowly, carefully, and gently, you should have no problems.
You are absolutely correct that, in the event there IS a button that holds the case in place, you absolutely should have it depressed, but the button in this picture looks like a power button or something similar, making your point irrelevant.