Easy. They don't do it.
The best they do with IPs is check to see if it matches your "normal play location". Meaning they are seeing if you are botting or if your account was hacked and then botted by the hacker(s).
The never ban by Mac Address because they simply don't have that information (it's illegal for them to even try). The don't use Hardware ID because, again, it can't identify a person, is easy to change and again is illegal for them have/use.
That type of information (MAC addresses, HWIDs) is strictly personal data, and it's highly confidential. Because of this if Blizzard would even gather and/or use it against you, aka for identifying all the clients ran on your machine, they will break a lot of International and US federal laws in the process.
Botting accounts being banned is literally ALWAYS related to unsafe practices by the botter. This can be as simple as accounts being linked via in-game transactions with accounts selling gold/botting. Excessive time periods of botting (like 24/7). Or botting "stupid things" like BGs, which can be quite obvious and is likely to have people report you. Or if you bot obvious or well known routes or areas. Even when we have tripwire events some people will still keep botting depsite it not being considered 'safe'.