@janneke @jesterchen @actuallyautistic
I didn't "want" this question answered one way or another. I legitimately wanted to know if it was just me that thinks it is insanely rude. I worded it as such. Turns out, I'm with the majority for once in my life.
The reason it is rude is because of the effect speakerphone have on conversation volumes: It drives them up. If you were just speaking to a friend close by, you would naturally(hopefully) adjust to each other's volume levels and the ambient noise level and in all likelihood(again, hopefully), keep it down. When you use speakerphone, it comes through loud and clear for everyone in the vicinity, this also has the impact of making the local phone operator tend to elevate their own voice.
So, while using the earpiece would result in everyone nearby hearing little more than your likely hushed voice. Using speakerphone in the same situation usually means we can all hear every word of both sides of your conversation loud and clear. Speakerphone causes you to be loud in public. This is why it is rude.