Once upon a time, that was the rule.
It gave rise to a phrase, "Move up, move up, move out". Which applied to players who went pro prematurely, but were never really that good and plateaued at the bottom of the pro ranks. So they were left with a choice -- keep finishing at the bottom and never win anything....or quit. They'd quit.
The words "pro" and "am" are overblown. McBeth is a pro. The 960-rated local who occasionally plays hot enough to win his entry fee back, isn't. So for the same reason we allow amateurs who play an event above their rating to later play their correct division, we allow players who've cashed in pro to play am in subsequent events. With restrictions -- we don't allow top-rated pros to dive back into am, and we don't allow any pros to compete in Am Worlds.