If you are good with a flexible boning knife you can take it off with a little extra effort.
While it is true that the cure will not penetrate the skin, you simply adjust the cure time to accommodate leaving the skin in place. With proper curing time allotted, the cure and the flavoring ingredients will fully penetrate the pork belly.
During smoking, I can easily remove the skin once the belly nears 125°. It just easily peels off with little resistance.
I recently did 60 pounds of pork belly and I peeled it during the last stage of the smoke. The smoke fully penetrated the bellies even with the skin on.
Smoking until the internal temperature of the bacon is 125° is not hot enough to cook, half cook, or even to kill bacteria. But this helps to melt some of the saturated fat to concentrate the flavors of the bacon, as well as firming to facilitate easier slicing.
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