I don't know why this is such a surprise to anyone.
Since political parties came about, it was "the victor goes the spoils". Whatever party came in cleaned house, and this is in Federal, State, and local politics.
The development of civil service came about to keep at least some people on their jobs and maintain continuity. But all policy makers and high level people serve solely at the pleasure of those newly elected.
A very few people manage to stay on the job, some doing work no one else wants (ie Prison or Sewer commissioner, which aren't very glamorous).
Even within a single party, there's "punishment" for those who didn't support the eventual winner. Basically the life of a politician is filled with lining up supporters in a constantly changing world, and choosing who to support elsewhere. Alliances are constantly being made and then broken.
FWIW, when FDR was elected in 1932, Herbert Hoover was out of govt and persona not grata. But when Truman, a very loyal Democrat, inherited the office in 1945 he promptly involved Hoover back to the White House and put Hoover's considerable skills to work for famine relief in Europe*. On the other hand, Eisenhower and Truman had a bad falling out and Truman was not welcome in the house during Eisenhower's term. In yet another twist, Kennedy brought back Truman to honor him despite Truman despising Kennedy's father and being very against Kennedy to be the 1960 candidate.
The point is that politics is all about friends and enemies and reward and punishment, and it's an accepted part of the game.
(*Contrary to myth, Hoover did a lot more to fight the Depression than is given credit for.)
It's not much different in the private sector. When a CEO comes in there's normally a reorganization and he puts in his own people and others have to find new jobs, even if they've been with the company for years.