I agree, that reaching to people with your criticism when they don't ask for feedback, is not polite and maybe out of place. But if people ask directly for criticism, it means that they want it, isn't it?! Moreover, if people ask for feedback/review, that means that they also don't mind to receive some criticism as a possible part of this review. Because if they don't want any criticism in the feedback, they would ask for positive feedback specifically or just would go to corresponding positive-feedback forum threads.
I fully agree that people can take thoughtfully only restricted amount of criticism at a time, and that novices and young people should be treated softer, than mature and experienced one. I also agree that it is good practice to always try to find a good aspects in people's work, to not making your review too demotivating by the chance. But... praise the work which you don't like at all? Hmmm... 
I personally don't want other people to praise my work if they don't like it. If people don't like it, but don't want to criticize particular aspects in detail (or I myself don't ask them to do it), they always can say that it's just not their cup of tea, or something like that. It may be sometimes a bit sad, but totally not insulting and fully understandable and normal.
It's your deal how to live and how to communicate, guys, but... would you want to others to do the same to you? I mean, do you want others lie to you that they like your work, when they don't?
I don't. Moreover, I would be really mad if I'd realised that someone lied to me like that because of pity, while actually they think that my drawings or stories are garbage. It is much, much more humiliating than just saying that they don't like it.
From my own experience, blatant lie is very problematic and hard to be keeping undiscovered. And when it get undiscovered, it may hit hard. So in my opinion, it is more simple, convenient and safe for everyone to just don't produce unnecessary lies and keep proper degree of honesty.
P.S. Also after these comments, all creators, who have unstable self-estimate (including myself), have a chance to gain a paranoia that people who have praised their comics, have lied. 