Hmmm...... You are right, of course.

Could be that the island does not permit inflicted injuries to undo what it has remedied. It made Locke able to walk and won't permit anything (other than itself) to change the arrangement, so it healed him -- really quickly!!!

Recall when Boone had to scamper into the Beech Craft because Locke's legs weren't working well? The island must have the power to alter its effects.

I'm grasping here, okay???