My sense is that the way bloggers "ruin" flyertalk is that they make access to inside information non-reciprocal. FT is a participatory community, in that access to deals and information is implicitly conditioned on participating in the community. People here share information with each other with a good faith expectation that others will do the same. Bloggers destroy this expectation by making the information available to people who do not participate in the community.
Of course FT lurkers don't really fit into this paradigm but FT has a pretty steep learning curve that diminishes the value of casually lurking. In order to benefit as a lurker you have to do a lot of it so that you can learn the jargon, separate the wheat from the chaff, etc. This involves a heavy investment of time which discourages the practice. What the blogs do is drastically decrease the cost of obtaining this information.
This reduced information cost allows many more people to enjoy the benefits of the information without having to participate in the community. I think this is where the feeling that FTers are entitled to deals more than outsiders comes from: FTers have spent a lot of time developing a collaborative community that encourages reciprocal information sharing for mutual benefit, and the bloggers are profiting themselves by allowing people to benefit from FT who have no interest in making the time-investment necessary to reciprocate. It's a one-sided exchange.
I'm not convinced this frustration is justified, but I certainly think it's understandable.