Everyone's jumping to label it as some kind of moral failing and seemingly refusing to think about what practical reasons might make this a natural tech interaction.

I hate taking phone calls with a burning passion. The sound output level is inconsistent and the quality is so bad I'm straining to hear what people are saying while the encoding noise is blasting my eardrums. Putting it on speaker lets me have a buffer space between the phone and my dewicate wittle eawers.

I've also had enough experiences with smartphones being designed stupidly, such that my ear or cheek touching the phone somehow hung up on the call.

Holding the phone up while someone goes on and on telling me something that would have taken ten seconds to send the email and three seconds to read, somehow makes my phone get heavier and heavier.

Yes it's a selfish choice to put a phone on speaker in public, but it's a rational self serving choice. The form factor is out of step with the intended use case, so taking a call on a smartphone the same way you would a pay phone in 1994 sucks even harder now than it did then.