I honestly don't think Up Front kicked Sayuki out. I do believe it was her own choice, but don't think it was based on free will. She obviously was pressured by the industry traditions and standards.

The fact that Saori is still around proves that they get a choice, and there have been plenty of scandals before that where nothing has happened
Mai and Chissa literally had one each a little while before °C-ute announced disbandment (two or three months earlier iirc?)

Sayubee's situation is very much higher profile (first, getting bunshun'ed and second the dude being famous) and that put her on a terrible place imho. She probably felt guilty because she is "representing H!P" for the general public in a very public way and it's not a "good look

I'm still livid that she feels like she has to quit. The fact that these industy standards and traditions exist and probably made her feel bad and guilty enough that she had to quit sucks.
The fact that there are fans that are mad at her and happy she quit sucks even more.