A few bad actors with duplicate accounts could potentially overload the infrastructure we have set aside for free tier customers, which could degrade the experience for legitimate users or increase our infrastructure costs to a point where it is not economically feasible to sustain a free tier. We abandonded the one-account-per-IP rule in 2017 and switched to requiring all users to verify their email address and using block lists for disposable email address providers, which were common among bad actors (but also some legitimate users...more on that below).