It's call TOP TUBE FRAME PAD or TOP TUBE PAD.
These were first seen on BMX bikes back in the 1980s, and did about as much good than as today. A thin foam cylinder wrapped around the top-tube does little to protect the family jewels from a whack, but it does at least protect the knees from a knock when you bail on a 360º endo. For a fixie, though, it is nothing more than posing — if the riders were that worried about safety, they’d buy a top tube pad. price around $10-$30 (Not exactly).