I think it's common knowledge that the actual ceremony are limited in the number of tickets. I still don't think that would warrant an annoucement. Certainly everyone that you'd invite to a party wouldn't be invited to the actual ceremony. Thank God..

I mean while certainly... it's an important day to the graduate (and we've got one this year) and their family, most folks coming to the party don't want to be at the actual ceremony...with the possible exception of grandparents and the like.
Then again, I imagine their might be some folks
relieved by the announcement and not the invite...