I'm a SONE of 11 years, have gone through multiple events and attended few concert of theirs, and surely as hell i was there during 9/30
It's so evident that she's just one selfish girl who wants things to go her way. She clearly stated how she missed some training time/schedules of SNSD because of her business and you know damn well SNSD was busy preparing for their Tokyo Dome concert around this time. Her justifiying other members' absence made her look like an ass, missing schedules under SM that have been properly designed to fit into SNSD's training time for Tokyo Dome is much more reasonable than impromptu travelling schedule with your boyfriend for solo business.
TBH I would get why her actions would get on the members' nerves. SM clearly stated that THE BEST(Japanese Version) would be her last active period with SNSD & Tokyo Dome would be her end game before she go into hiatus to focus on her business. The WORST part is SNSD members agreed to this and wanted to send her off with Divine & ITNW (Ballad Version). The members honored her legacy towards SNSD by wanting her to be a part of the group's biggest dream at the time, performing at Tokyo Dome. The Catch Me If You Can EP was supposed the be her last album with SNSD before her hiatus
BUT NO she had to ruin all this because she can't properly handle herself. Tbh this can all be EASILY avoided if she decided to start her business AFTER her hiatus begin. Isn't it a common knowledge that starting a business would require A LOT OF time & in her case, travelling to meet her business partners. She specfically wrote in her 9/30 letter that she couldn't back down from her business because she had ready signed contracts with her business partners. Why the f would she set up her business on one of SNSD's busiest year, while still wanting to be a part of the group AND you are going to be on a hiatus after the Tokyo Dome concert?
Her book, actually answered everything to me(you need to read between the lines).
But it doesn't make SNSD's part of the story fully saint. After their argument in 'Shine' and Jessica picking to stay in the group & run the business, they voted her out(since that was the only way for them to be sure about where they are standing for their Tokyo Dome concert) but not even ONE person contacted her and poor girl had to know this in NYC, in a hotel room with her boyfriend?!
I totally understand the emotional damage that have on Jessica, who actually treasure her time with SNSD and wanted to give her best to the group. They should have allowed Jessica to join the China fanmeeting and have one last discussion(INCLUDING her) then decide to either continue as 8 or let her be(I believe this would have happened since Wonder Girls faced the same situation with Sunye)
TL;DR: The reason behind her removal was justified in her book(Jessica), the way it was done was messy as hell and no one to blame for that than SNSD.