Sadly, when it comes to manipulation, human kind has already lost this battle.
> improve the public impression
Someone trying to influence your impression IS manipulating you.
I've spent 5 years studying psychology at uni so even though I don't consider myself as an authority, I think I've got enough knowledge to know what I'm talking about. I don't give a flying fuck if the person reading my comment will be convinced or not. With that said, if they at least do a research on the subject, we'll all be better off.
> In contrast to your comment, an ad for Viagra is straightforward
Care to elaborate a bit?
How discussing with someone anonymously on the internet compares with a situation where a company is using all kinds of techniques to trick people into spending money on their product?
They use all kinds of sophisticated socio-psychological techniques to do so.
Unfortunately, most people (including yourself) don't even realise this, hence they become more prone to such techniques.