Why does the TO have a Close button even if it is fully received?

I don't have a NetSuite authorized answer but in my experience a "Fully Received" TO reacts as a "Closed" one would in terms of filtering to only "Open" TO. There is likely a difference as "Closed" on a TO means that each line item is "Closed" which NetSuite takes to mean that it did not go through and post the accounting impact for that line. "Fully Received" means that the TO was processed. Think of the difference between a "Closed" sales order and a "billed" sales order. Both are technically closed but the 2 statuses are different as one posted and the other didn't

Thank you. If I'm not mistaken, there's no Close button anymore if the SO is fully billed and fulfilled.

Ah, yep, you are correct. You're right that is odd that there would be a close button even after a TO is fully received.