The freezer, working or not, would be just about airtight, and so would have been a way for the living to separate themselves from the dead inside the dome.
Where else to put the dead? Outside to be eaten by dogs? Among the people in the arena? No one was about to live in the freezer, so it would be a logical otherwise unused and airtight place for them to put the dead.
Also after the power went out, the freezer was likely to stay cold for some time--perhaps for as much as two days it would have been cooler than where the living were.
And since people probably believed they would have been rescued before the freezer warmed up, they continued to place the bodies in the freezer in the hope of preserving them until rescue arrived.
I can imagine that at some point it would have warmed up and the decay would have started--perhaps then no one wanted to open the doors to face the concentrated smell inside, and so any subsequent dead would have been placed outside.