Because your whole foot moves. When you pivot 180 degrees, no part of your foot is over the same part where your foot was. Pivoting is allowed on the balls of your feet. When you do that, your foot just turns and most of your foot is over the top of your old foot print.Why do you think pivoting on your heel is a travel?
Had a teammate, who our coach had to bench since he got called for travels in consecutive games many times for it. Spent a week working on his pivoting to stop the travels.