It's ok if both parties know what's going on. If visitors are left with the expectation that a message becomes visible on the other end when they send it then it is not ok at all.
You lose so much with typed text which makes good customer services a lot harder. Thus as long as any pre-posted text isn’t stored anywhere (I fully expect posted chat logs would) then I don’t see an issue with support operators using real time text as a glimpse into the customers mood.
[edit]
To the people who voted me down: I get that you disagree with me but I have been on courses regarding just this (which is weird because I couldn't be in a less customer facing job....) so what I'm talking about here isn't just some random junk I've invented off the cuff. It's what I was taught how customer services (the good ones anyway) work.
Anyone who's ever done a stint on 1st line tech support will understand just how much of a thankless job it is. If this helps them to serve me better then I welcome it.
I think the real complaint being made is that you didn't realise they were using these visual clues. But that's why I keep coming back to how other forms of customer services are trained to read vocal tones and body language. Would you also feel cheated if you learned your favourite high street store's customer services team had training on reading body language while you hadn't so they have an advantage in gauging your temperament but they didn't let on they had that training?
Don't get me wrong, I do see and understand your point. I'm very privacy minded so this is the kind of thing I'd normally get annoyed by as well. But at least this time the anti-privacy tools are genuinely being used to improve customer experience rather than just to monetise them (yes I know good customer experience can lead to repeat custom - but more often than privacy is sold to the detriment of customer experience)