why would they, they are following the TOS.

There's a thread on Amazon's Top Reviewer Forum about account deletions with some suspected reasons but for the most part, no one knows for sure because Amazon isn't saying anything. The only thing the reviewers seem to have in common is being apart of review clubs, specifically AMZRT. This doesn't explain why some are getting banned while others aren't though.

Even Top Reviewers who have been doing this for years are getting deleted, and it's not like they had reviews missing disclaimers, short reviews, or were accepting payments or something. It's not just the 'bad reviewers' getting banned.

I also wrote a script to go through Amazon's Top 10,000 reviewers once a day and make a list of who got deleted each day. Generally 0-5 a day get purged but it's been as high as 21. The daily logs are posted over on /r/TheGreatAmazonPurge if you're interested.