On AT&T's network at least, the iPhone, even jail-broken ones typically, sends its tethering traffic to a different APN (AT&T access point). Making it very easy to detect when an iOS device is tethering unofficially. Android doesn't segregate its tethering traffic like iOS -- which makes it much more cost prohibitive to detect its tethering traffic; consequently, because of this, and because the number of Android devices on AT&T's network is dwarfed by that of iOS devices, it's likely that Android devices will be excluded from the crack down.