But in many cases a server operator doesn't expect any legitimate traffic from that half of the internet, or is willing to block traffic from it.
For example, there's generally no reason a customer would use their internet banking app with traffic routed via a datacentre other than for the reason you proposed (masking their IP address), so if the bank wants to prevent people doing that then blocking all data centre traffic is an effective way of doing it.
For example, there's generally no reason a customer would use their internet banking app with traffic routed via a datacentre other than for the reason you proposed (masking their IP address), so if the bank wants to prevent people doing that then blocking all data centre traffic is an effective way of doing it.