VPN providers don't have unlimited IPs. At some point, you've blocked all the VPN IPs, and suddenly it gets much harder to abuse the site.
If this doesn't solve anything in your eyes, what exactly should a provider do when they're getting hammered by VPN IPs? Should they tell their customers "sorry our service is offline, but there might be people legitimately attempting to access it through the IPs that are DDoSing us"?