Why would there be a lot of libertarians in the ACU? There is another libertarian on this board, I think that it is Jason Havestdancer, who posts everyday that libertarians and conservatives are completely different.
The ACU has always been a bit more libertarian than the GOP generally. I recall that back in the early years of this century they started a discussion board. I was a member of it. George W. was president, but the posters on the board were overwhelmingly anti-Bush. About half were against the War in Iraq, and the other half were against No Child Left Behind, the prescription drug bill, and other Bush spending programs. The ACU was so embarrassed by this that they shut the discussion board down.
Aside from that, it is also important to note that the average age of ACU members is quite a bit lower than most other conservative organizations. A majority at CPAC, for example, also favored the legalization of marijuana.