You proved a nice theorem, wrote up the paper and submitted it to a major computer science conference. Your paper was accepted! Congratulations. Now pay up.
An author of every paper accepted at a CS conferences is expected to present that paper at the conference. To do so requires at the least paying the registration fees, travel and lodging to go the the meeting. That can easily run one to three thousand dollars (or more) depending mostly on how far you need to travel.
You can use grant money for these expenses. Some conference offer support to those who need it, particularly students. CS departments will often help out if needed. Sometimes people pay out of their own pockets and, in any case, the funds come from limited pots that could have been used for other purposes.
In the "old days" this was less of a problem. There were only one or two conferences relevant to one's field and you were probably going to those conferences anyway. Now as the field has grown and it has been harder to get your papers published in the strongest conferences, you may find yourself traveling just to give the talk. Even many major conferences don't draw many attendees who don't have papers in the conference.
We haven't seen an outcry of these expenses, say compared to the outcry over the cost of digital library subscriptions. Perhaps we consider attending the conference a "reward" for getting published.
We could just eliminate the conferences and publish the proceedings and post videos of talks, made at the home institutions, saving the field huge amounts of money. Then we could actually choose to attend conferences to meet people in our field instead of just talking at them.
Note: Elchanan Mossel had a similar observation in a comment on a recent post.