The thing is, logging into a network domain (Windows) on an iPad isn't as useful as it is on a desktop or laptop. You can't browse for files, launch enterprise apps, or use internal enterprise websites.
What most business networks do instead is provide you with access to things like your email, outlook, and perhaps a few specialty databases (through apps). Your apps contain the necessary username/password, and come into the the network from the outside; perhaps using a VPN (virtual private network).
Enterprise networks usually install a profile policy that enforces minimum security settings. You never log into the network the way you do with a computer. The keeps anyone from just joining the network with any old device they happen to have.
That being the case, you can use any internet connection. But it has to be set up by the network administrators.
At least that is my limited understanding.