Nike dropped Maria Sharapova when she tested positive. Did the same with Marion Jones and Lance Armstrong...and they're like the heavyweights in their sports and some of the most high profiled athletes of their time. It would be a PR nightmare for a sports oriented company like that to stick with a "disgraced" athlete or be found interfering with an on going investigation to save their brand by covering it up.
Sharapova is still a Nike athlete, they just suspended the relationship briefly after she was banned. Though her situation is totally different to the others you mentioned as she only broke the rules during one tournament which she didn’t perform well in anyway, a relatively minor indiscretion as the CAS found.