I can't see any economic argument for banning insider trading. In general, the point of allowing financial securities to be traded is to ensure that they are properly valued. "Insiders" seem better informed about the proper value of these securities than "outsiders". In few other realms do we ban people who are knowledgeable about a subject from trading items relating to that subject.
Apparently the primary motivation for the ban is a sense that it's "unfair" for insiders to profit from information that other people don't have access to, but I don't understand this normative argument. We don't think that it's unfair for doctors to profit from their specialized medical knowledge - or for that matter, for successful CEOs to profit from their industry expertise by receiving high salaries (as opposed to making well-timed stock trades). A potential investment advantage from access to "inside" information seems like just another form of indirect compensation, which will be priced into the wage market and (all else equal) lower the nominal salary just like all non-cash benefits.
I'm not trying to make some point of libertarian principle or start a philosophical argument. Nor am I assuming that there's some heavy onus on the government to justify any economic regulation. I'm simply looking for a value-neutral summary of the argument for banning insider trading, since I've never heard such an argument.