This is because of product variants and the tree view is a list of product.product objects (variants), that column is a related field on the product.template (the parent product of the variants), you can't by default sort by related fields, so this problem is quite specific to the internal reference on the products list view.

To fix this you need to write a function called _generate_order_by on the product object (or override the product object to add that function)

There is some discussion of it here including some code for a similar field http://stackoverflow.com/questions/15621013/sorting-openerp-table-by-functional-field/15621014

I believe you can add the sort option only in class level, and not GUI level...