Can a network merchant has his own campaign categories?

Not exactly. This is because affiliates can see the categories. So if each merchant created his own categories, there would be a mess in the categories, as each merchant can create the very similar categories... Therefore this feature is only for network owner  to provide a clearer view for affiliates. If you allow network merchants (in permissions) to be able to modify the categories, it will cause the same mess, as each merchant can change any category - this means, one merchant can change "drugs" to "pharmacy" and then the first one modify it back to "drugs"... and that is not very helpful.