These are different levels of privacy invasion, at least in the US. Going through a phone to check what you are thinking / saying is seen as more intrusive. So

These are different levels of privacy invasion, at least in the US. Going through a phone to check what you are thinking / saying is seen as more intrusive. So even in a stop and frisk, going through someone's phone would not generally be justified. The rules here are very clear. In particular, even in a terry stop police generally are not authorized to seize / search your cell phone. But they can pat down your outer clothing for weapons.