Wasn't this posted already, don't know where I read it. But yes, a lot of websites are probably using this technique, so I've decided to always compose my message in a text editor.
I would imagine that retention of this information/data aquired in this way, prior to a conscious 'commit' is probably illegal. Your good suggestion to first use a text editor to compose the post prior to commiting to the form is something that I have been doing on and off over the years but with this new revelation, I must make it a rule to do this every time I will be using a submission form and including none commercial submission of any data in a form format. It seems that there is no end to the sneaky tricks to gain advantage that these parties will use to undermine the confidence in usual commercial practices and against public expectations of ethical behaviour. I think that companies that are cynically involved in these practices should be exposed.
I have been doing this for the last 2 years, I always had a feeling they were able to snoop on everything that you type. But it depends on the importance of course, on certain sites I don't really care.
I guess no one noticed, wilders does this behaviour too lol... Most noticeably when composing a new thread. If you have a network activity indicator you can watch it send data after every letter while you enter the thread title. I mean a real activity indicator not the phoney animation MS sneakily replaced the real one with some years ago.