I'm going to break down my opinion here into bullet points, since there are several aspects to this:
1. As both a forum member and a moderator, I think it is very important that all members feel welcome to express their personal experiences with stories, as long as they do so in a civil manner. As people who feel like wading through my posting history will no doubt be able to confirm, if I listened to a story and it did not feel like SF to me, I have pointed that out. I have no plans to ever deny anyone else the opportunity to do so.
2. However, there is a big difference between saying "This story did not meet my definitions of SF" and saying "this was not SF", just like there's a big difference between saying that "I did not enjoy this story" and saying "this story was terrible". Expressing one's experience and reaction to a story framed as a subjective viewpoint is a valuable contribution to a discussion and something that is of interest to the people involved with the podcasts. Expressing one's experience and reactions as if they were universal truths shuts down conversation and does not give the podcast editors any way to use your feedback.
3. Both Steve Eley and Mur Lafferty, in their days of editing the podcast, have stated that just because Podcastle and Pseudopod exist, that will not stop them from running non-SF stories when they wish to. Norm has yet to say anything overtly on this (or really, any) topic, but I don't think he plans to differ from his predecessors in this regard. If you sign up to any sort of edited fiction publication, be it a podcast or a website or a magazine, you are signing up to the vision of the editor. Unless you are the editor, you have to accept that sometimes their decisions won't be the same as yours. If their decisions often aren't the same as yours, then maybe it's time to abandon this particular editor's work. Insisting that the editor always make the same choices you would have makes no sense and will not get you anywhere.
4. That said, there is a place for feedback on editorial policies as well. But it's important to realize that feedback on editorial policy is not the same as feedback on an episode, and just because an episode is an example of an editorial policy you dislike (for example, because it is not SF enough), pointing that out is still arguing about editorial policy, not about the episode. I'm not going to get into this in depth because I've posted on this recently in a different thread, but as a moderator, I will intervene if I feel an episode's thread is getting dominated by general editorial critique.
5. Sometimes, interesting criticism becomes less interesting because it is framed as a genre argument. Take Mat's example above: "this is not a sci-fi story, this is a story about people that happens to take place in a sci-fi environment" - that's a rather pointless thing to say - as I said above, it doesn't really give anyone anything to work with. Compare this to "the story's setting is not really related to its themes, and the two distract attention from each other". That is something substantial to say about the writing of the story, and is a basis for a discussion. It could be what the first statement was trying to say, or maybe not, but it's definitely a lot more interesting.
6. As a summary, I disagree with Mat's assertion that the inherent goal of a critique is to suggest how something be improved. The inherent goal of a critique is to exchange opinion and discussion of what a work of art's effect was on its audience. This may lead to something being improved, or not, but critique is inherently part of a discussion. The discussions on these forums aren't always perfect, but they're at their best when people post not just because they want to state teir view as strongly as they can, but when they're both open to being exposed to other views (which doesn't mean that they'll be convinced, but that does mean they'll take them seriously), and are willing to state their own views in a way that encourages discussions rather than attempts to win some sort of non-existent competition.