Most sites like Alldebrid, Wikipedia, etc. ban VPN logins. By "banning" I mean that the site doesn't open up all of its features to users who come in from a VPN.
So why does Stack Exchange still allow VPN logins?
I assume you mean public VPNs, since a whole lot of us use VPNs for work...
Anyway, the answer is "because it doesn't matter". To the extent that we use network-based blocking to reduce abuse, we use it for all networks - after all, abusing the site from your desk at work isn't somehow less abusive than doing it from a coffee shop over a VPN.
If someone wants to pay money for a different IP that anyone else with money can abuse and get banned with no recourse, that's on them. Same with Tor and other proxies - your access is perpetually at the mercy of whoever else is using them, and vice-versa.