I don't have some special personal reason. Simply that the numerous times I've seen people use the term 'sweetie' or 'sweetheart', in an online setting to a stranger, they have been Americans.
Brits may use terms of endearment in real life (yep, you'll get shop assistants calling you 'love' or even 'lover'!) or to people online with whom they've become friends, but I have never seen them use such terms online to relative strangers, and certainly not in the specific debate/disagreement type context I described. And obviously as a Brit, I tend to be aware of why and how my fellow Brits use terms of endearment, so I have no reason to ask about British usage of the terms.
So I am asking about American usage, because it differs from British usage, and I'm curious what it means. I would ask about Australian usage, but I've never ever experienced Australians using such words - they're more likely to call you a dag, and only when they've got to know you! ;D