The Linked Data FAQ suggests that the 'open' portion has to do with licensing:
Does all Linked Data need to be Linked Open Data? In a word, no, and it will likely never be so. The label "Linked Open Data" is widely used, but often to refer to Linked Data in general, rather than to Linked Data that is explicitly published under an open license. Not all Linked Data will be open, and not all Open Data will be linked. Therefore care should be taken to use the appropriate term, depending on the licensing terms of the data in question.
(Contributor: Tom Heath)