It doesn't matter what the package says. When you buy something, you should look for signs of tampering before you check out. And when you film yourself opening a sealed box, and something goes wrong, now you have evidence. Laws supercede store policies. Most countries laws would allow you get get back your money especially with evidence.

Maybe if you open like a cereal box, you don't need to bother. But opening a new smartphone? I think it's worth the annoyance of filming yourself opening it. If you don't film it, best case scenario, you save a little bit of time, worst case scenario, you lose out on hundreds of dollars because it was damaged or missing.