Personally, I welcome any criticism in the comments of my hubs as long as it is constructive and expressed in a mature manner. While I can't vouch for anyone but myself, I imagine there are many here that share this sentiment.
However, I can also understand why some hubbers would prefer to keep their comments clear of anything that they would consider negative.
Depending on the author's self-esteem, constructive criticism could be interpreted as a personal attack. Some people are here at HP to share their stories and activities and aren't really looking for advice.
For those who receive a lot of external traffic, they might choose to hide anything that isn't a glowing recommendation of their hub in order to establish authority and hopefully convert the traffic into revenue.