But coming back to freebitco, if they know making use of ip addresses will pose such issues, why then didnt they make use of device ID instead? I think this is more unique and would help in reducing such errors like users being told that someone else claim their free roll with the same ip, isn't using device ID more feasibility and reliable than use internet ip addresses? What do you think?
IP restrictions are the easiest way to mitigate multi-account abuse. They do also take other factors into account to assign you a fingerprint and stop more sophisticated forms of cheating.
The IP warning is trivially bypassed by using a VPN or proxy. If you are not trying to cheat, you are not likely to face an issue when using a VPN, although you might still encounter the warning because VPN IPs are typically shared by many people.