Markus Ruhl does not get booed. He has told his fans, both before and again at the press conference, that he loves it and gets his energy up when he goes on stage and the fans shout 'Rooooooooool'. It is pretty cool to hear it, and it is pretty loud.
In fact, so customary, that whenever Markus is around and they announce him, the fans say 'Rooool'. It is pretty funny, and this has been going on for years now...