In general, operators either want to either SIM lock the device after subsiziding, but equally well you can just have a fixed 24 month contract for your SIM card and not worry about locking the device: the consumer pays the same whether or not he uses the SIM (and/or the device). That works well if it's a relatively fixed flat monthy fee. For deals where the price depends on the usage, SIM locking is preferrable to operators, since you don't want the customer to use the device with a cheaper deal.