Because it's still below the DC minimum wage, and because it's impossible to survive on $8 an hour you absolute chode.
You don't understand how this works. Waiters must always earn the local minimum wage. If the base pay ("tipped minimum wage") plus tips received does not equal at least the local minimum wage (averaged over the time covered by one pay period), then the restaurant must make up the difference. If a waiter works for one pay period and gets zero tips, he will still make the minimum wage because the restaurant is required to make up that difference.