Why not simply charge for access to the forums, like they do on other IBB's?

You'd then reduce your membership base which probably factors into selling ad space here. In the final analysis, the loss of ad revenue due to the smaller numbers of members which would result from charging access/requiring an InsideFlyer subscription would have to be made up by the membership which remains, though, IIRC, I don't think FT was meant to be a money-making endeavor.

Also, consider that any move which results in a loss of membership would probably result in a loss of value for what makes FT so great: miles/points information.

It's a d@mned-if-you-do/d@mned-if-you-don't situation.