Okay, this is misleading. They received beyond their normal salary the government Per Diem, which amounts to $8/ day. Also, this was reduced somewhat because they didn't need to pay for their sleeping locations. This is the same as any government travel, if they were traveling in a location where they didn't need to pay for their housing, they didn't receive that portion of their Per Diem.
FYI, the system is still the same today. For government employees to travel (And for those working government contracts), they receive a portion of their Per Diem for a hotel, and a second portion for their food. In fact, I suspect it was a bit of a breach to not also charge them for the food, giving them just their normal salary.
For reference Buzz Aldrin's Per Diem for his mission to the Moon comes to $33.31.
But they did receive their normal salary.