Although, on the entertainment side, there is the recent, semi-recent case of CM Punk and another wrestler that were fired from WWE and it was revealed they had a 1 year no compete clause that prevented them from joining another wrestling or MMA promotion for that period. Both wrestlers had the money and took WWE to court and won, and CM Punk is currently training for his first fight in the UFC.
Yes, Vince McMahon is famous for this kind of stuff. That's why everybody hates him. Plus, wrestling fans are not known for their discriminating taste and intellect. So maybe that's why not all of them have left the WWE yet (though most have).
Though I'm curious what artists you'd say H!P has held back with a no compete clause.
Maybe Gocchin when she moved to AVEX, but that's about the only example I can think of
It's difficult to know, because the Japanese entertainment media is too obsessed with exposing idols' personal lives to bother doing their jobs and investigating how the business works behind the scenes. (maybe because fans don't really care how the talent they watch is treated behind the scenes, which is sad).