Ed McMahon, Top Second Banana, Dies
The reason for not comparing people to Tonto is that Tonto was the white man's stereotype of what a good Indian should be. Tonto the fictional character may have chosen to be a sidekick, but Tonto the real person probably wouldn't. He'd be wondering why they were catching outlaws while US soldiers were massacring his fellow Indians. He'd be asking himself if he shouldn't join up with Crazy Horse, Geronimo, or Quanah Parker.
McMahon was subservient by his own choice. Tonto was subservient by his white creators' choice. Comparing McMahon to Tonto implies Tonto was happy to be subservient. That it was a natural and healthy choice for an Indian like him.