I use speaker phone in public
Why do you want everyone to hear your conversations?
Talking over speakerphone is more direct, so I say "Hi."
Using a phone in pubic its expected that you do so privately. Other people around you may not want to listen to your conversation and shouldn't be forced to. Otherwise they may become annoyed.
It's fine since my voice is not loud. I never shout.
It is distracting to other people, even if you are being quiet. It is rude to do things that you know is going to annoy people.