By dint of not adamantly refusing to deny such theories when asked about them.

A lot of you may be wanting to say “But he never explicitly endorses any 9/11 conspiracy theory,” but that doesn’t matter.  The point is that he was asked about it, and instead of saying “those theories are absurd, the twin towers fell because a bunch of Islamic extremists crashed planes into them” he goes the “well I’m willing to support any theory based on fact/I haven’t heard enough about the theory to make a decision” route.

Which is a total cop-out aimed at pandering to the far-left lunatics who perpetuate this nonsense.  And if you’re not with me on this, ask yourself how you’d feel if Kerry had made the same response to a question about some study done by an obscure college professor purporting to prove that blacks are racially inferior to whites.