I find the only site owners who block proxy sites are usualy small time site owners near or around 100,000 Alexa rank doing it out of some personal reason of being against someone being able to be anonymous on their site. Sites like Facebook don't block proxies. You would think they would a troll epidemic?
Spammers and bots dont come threw datacenter ranges, Datacenters have to hold accountable of whats going with their network and would not allow that, hardcore Spammers use Socks proxies from places like
http://proxy4free.com/page1.html . Majority of those aren't from Datacenter IP ranges. There from accidentally left open proxies on residential IP addresses.