With DPI and other methods it would be possible for the carriers to identify and block most tethered traffic. From what I've seen none of them are doing that yet. If you can enable your device to tether technically you can do it but unless you're paying for a tethering plan it is breaking the terms of service you agreed to. Most carriers are now at least identifying tethered traffic and warning customers. Some do it simply guessing on data usage numbers and some are using DPI or other methods to be more precise.