Did you purchase the iPad directly from Apple, a third-party Apple retailer, or from a Cellular Carrier such as Verizon?
WiFi+Cellular model iPad's that are purchased directly from Apple are not locked to a Cellular Carrier. By contrast, when purchased from a Cellular Carrier, the iPad may have a subsidy-lock to the specific carrier. Subsidy locks are used where the purchase cost of the iPad is heavily subsidised to reduce its initial cost - and is effectively tied to a rolling Cellular Plan for at least a minimum period. Subsidy locks can only be removed by the Carrier to whom the device is locked.
If purchased from a third-party retailer, you may be purchasing a device that may be fully unlocked, or potentially subsidy locked.
Where subsidy locks have been applied, Apple cannot remove the lock. You will need to contact the relevant carrier for the lock to be removed. There is often a charge levied for unlocking unless the minimum Cellular-contract period has been fully paid.