If an employee has a legitimate reason for the absence, the agency could place the person on furlough, but AWOL employees can face appropriate consequences based on the discretion of the agency.
Gee, what could a federal employee use as a legitimate reason for not reporting to work, when they are being ordered to report and not actually getting paid for coming to work. Hm, let me think, let me think. Oh, I know! How about Amendment 13, Section 1 of the United States Constitution?
1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
I guess being essential, working without pay, for the United States Government, in the event of a shutdown is…punishment for a crime?
Failing that, not being able to pay for gas, or transit is also something I would accept. After all, many TSA worksites are not exactly conveniently located to public transit.