These completion ceremonies are simply fancier end of the year ceremonies than the younger grades get at my school. It is the parents that call it graduation - not the school. In addition to the regular end of the year awards (Honor Roll, Citizenship), the school recognizes school leaders. For example the specials teachers (PE, Music, Art, Computers, Library) give awards to students that have shown growth over years.
At the end of the ceremony, they walk through the school and the younger grade teachers/students get to say goodbye also. If a parent doesn't want to participate - the child simply doesn't attend that day. (basically the start summer vacation a day early).
At my school the parents that treat this as a "Graduation" with large numbers of family members coming to the celebration, balloons, a party tend to come from a particular segment of our population. Many of them are immigrants, who because of conditions in their childhood country they didn't get to go very far past 5th grade. It is their way of sending their kids the message this is important - education is valuable.