R2 demonstrably has rocket engines at this point in the movie. Yet he chooses to plummet uncontrollably across the tilting hangar bay, ending up crashing dangerously into a pile of rubble after a rather long fall.
And to address the obvious counter-argument: nothing ever runs out of fuel in a roleplaying game. (In fact, in the film, R2 does use his rockets again briefly in a later scene of this same battle sequence, demonstrating that he's definitely not out of fuel.)
Palpatine: Call the lift, Anakin.
Anakin: Um... we might have a problem. The operator droids have been killed in the fighting.
Palpatine: All of them?
Anakin: I have a plan. R2, can you hack into the gravity generator?
R2-D2: Of course. What do you want?
Anakin: Turn the gravity at 90 degrees. Slowly.
Anakin: Trust me.
R2-D2: Ah, I get it. Okay, one sideways ship coming up.
R2-D2: I use my rocket engines to stay near the data port.
GM: Sorry? Your what?
R2-D2: Oh... that's right. I, er... slide uncontrollably down the slope?
GM: Very well.