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A) We're currently checking a non-existant
variable_get('webform_enable_fieldset', TRUE) variable. But since it defaults to TRUE, the option is always shown.
B) Even if this were checking correctly, we shouldn't hinge the "Parent" functionality on fieldsets alone, since other component types can providing parenting ability (such as vertical tabs added through Display Suite).
So overall the fix is quite simple, just remove the check entirely. I switched the is_array() check on the same line to !empty() to work better in situations when creating a new node that doesn't yet have $node->webform['components'] set at all yet.