Since there's a lot of discussion about the Public Suffix list, let me point out that it's not just a webform where you can add any domain. There's a whole approval process where one very important criterion is that the domain to be added has a large enough user base. When you have a large enough user base, you generally have scammers as well. That's what happened here.

It basically goes: growing user base -> growing amount of malicious content -> ability to submit domain to PSL. In that order, more or less.

In terms of security, for me, there's no issue with being on the same domain as my users. My cookies are scoped to my own subdomain, and HTTPS only. For me, being blocked was the only problem, one that I can honestly admit was way bigger than I thought.

Hence, the PSA. :)