I was going to make the point just made by WB SATCO, but he got there first. If you fly to a field such as North Weald, Duxford, or Kemble, self briefing should indicate that some aircraft may be practising run and breaks. They may be normal (or, at least, not unusual) in a particular environment, and, as pointed out by several others above, are not inherently unsafe or inconsistent with GA. I agree, by the way, that although fun to watch and to do, they are not always needed by, for example, big pistons such as Warbirds and Yaks.