At most CS conferences (save some of the biggies like SigGraph, SuperComputing), at least half of the attendees are the presenters or co-authors. Those ticket prices are intended for you. It's not an accidental oversight. Their funding for the conference takes into account the speakers paying.
Out of the 10 conferences I spoke at in 2010, they were the only conference who made speakers pay. Also, many of the conferences I attended covered the ticket price and subsidized part of the lodging and transportation for speakers.
That said, all of the conferences I attended were very similar in size, sponsorship, subject matter, audience, and price, so this may just be a result of which conferences I attend. (In which case, I count myself as fortunate to find generous organizers.)
