Typically the iPad is not locked, regardless of purchase method. That doesn't mean to say it's not possible.
My suggestion as to why the carriers would not bother is due to the revenue stream.
If I buy a phone, the number I use is important, so remaining available overseas on the same number generates additional (premium) revenue. Conversely, the number assigned to the iPad is irrelevant, but I'm not going to make or receive calls.
If I change the SIM in my phone, the carrier 'loses' money. On a tablet, I, and I suspect many, will not pay for data roaming. So locking is not such a priority.
Two contradictiory arguments but both support the general practise of not locking the iPad.