In Pixels, in all other levels/games/whatever-you-call-it the aliens created 'the game' and the Earthlings were supposed to play the game. So why during the game of Pac-Man was it the other way around with humans playing as ghosts, and Pac-Man being the enemy? (Side question - why the cheat codes worked against the main player in this case)
Considering the plot tells us that they would have to work together to win, we can conclude this decision was made because in the film there are more than one hero (Brenner, Eddie and Ludlow at least). The only way to play Pac-Man as a team is being ghosts.
Extrapolating, this also could be an alien strategy considering that the original ghosts has a known pattern (at least for Brenner, who always played considering the game patterns). Playing as ghosts would be something new to them.